A Science of Qualia to Replace Conscience and Intuition

Conscience in Islam is defined as something that every human has been endowed with, and this makes it fair-game for Allah to judge at the end of times. Similar emphasis on conscience exists on some branches of Christianity, Buddhism, and Hinduism.

But what is conscience? It is often described as a spontaneous moment of recognition, an emotional whir that manifests at particular events that the human agent perceives bifurcate towards right and wrong.

There is a set, Ω, of possible worlds in the mind, and instead of reasoning about the probabilities of finding oneself in a specific world, given certain actions, the person trusting conscience will do what is compelled by a seeming god-given or nature-given intuition.

This sense of knowing right from wrong is a double edged sword. It can shield one from siding with Big Brother, religious dogma, or non-memetic genetic drives that lead to wrong. But it can also be the very same kind of inner-voice that tells the Chinese government official that he is right to torture you for having committed the crime of threatening the order with public dissent.

Instead of trusting our intuitions, we should look at the valence object, ω, that we wish be produced and calculate the probability that this object will appear given a certain action, P(ω|a).

Valence objects are subjective slices of now as defined by neuroscience and cognitive science. The problem facing us is that we do not have a catalogue of all valence objects. To catalogue them, we will need to capture both their physical isometry (neuroscience scan→bio-quantum-chemistry model→fundamental physics model→math structure) and their judged value after direct apprehension.

Right now, we are not even at the high-level biology understanding of valence objects in our day-to-day lives. We are not far beyond the literature/poetry level. There exist words that we combine to refer to what is good and what is wrong, but it is impossible to specify to another brain how to simulate the experience we wish to create for them past a certain rough-grain threshold. The qualia contents must be unzipped, transcribed, and translated in the self-reflexive entertainments of the other’s mind.

The future naturalists, those who stand a chance of becoming Darwin, are those who will venture into the sea of the mind to build a taxonomy, to anchor the hues of consciousness with scientific and mathematical tools. Once the nature of valence objects is described and explained, there will no longer be a need for ethical systems of old. There will only be a navigation problem, with fixed stars in sight.

None of this is to say that the symbols should be confused with the objects. An ISBN represents a book but is not a book. A catalogue of experiences specified by highly-precise physics is not the experiences. The experiences actually need to be instantiated on the necessary substrates in the 4-d flesh of the universe.

It would be great if there should be principles, mathematical symmetries perhaps, that underly pleasures and pains, and are generalizable across species. This way, “magnets*” in morphological space could be placed on the path of living beings without having to eliminate them all for the sake of a monotone hedonium nuke. *[Concrete examples of magnets would include neurosurgery, brain-computer interfaces, genetic engineering and virtual reality environments.]

But what if eliminativists are right and we cannot trust our own introspection whatsoever? In that case, I agree that a science of consciousness and morality would be doomed. But the hard eliminativist position which claims that we are deluded about our own experience and so our judgements cannot be trusted as more than relativistic noises, is, to put it mildly, absurd. One wonders how the hyper-skeptical eliminativist deduces anything about anything – even that there is such a thing as a natural world requires that judgements about experience be made.

There are no doubt going to be dishonest reports about the contents of conscious experience that will cause problems in the first stages of developing this science. For example, sometimes people give dishonest reviews about how enjoyable a book was for nefarious, profit-seeking motives. Or someone can claim, and later come to believe, that a meal was more enjoyable than it really was, simply because of their kindness and conflict-avoiding disposition. This seems like an insurmountable obstacle for qualia-science only to those eliminativists and reductionists who are but dualists at heart.

If we accept that valence is a natural phenomenon, then clearly this can be assigned values. RGB and “how much pain do you feel on a scale of one to ten?” actually refer to natural objects in the brain synonymous with conscious percepts. There is no extra illusion (what a sophisticated dualist calls it), or soul (what a naive dualist says.) There is just the universe in its totality, with certain chunks of it feeling like something unto themselves, and others not. The recollection cannot be perfect. For by necessity, it is its own region of the universe. But the claim that all information is lost from one moment to the next will be cast to shame by the foundational structure of reality itself. Hence, with meticulousness and by working from low hanging-fruit like orgasms, which are widely-regarded across the entire animal kingdom as enjoyable, for obvious evolutionary reasons, we will then isolate the substructures of the experience that are common not only to all reproduction-oriented pleasures but that also appear when eating a good meal (and are less apparent when only faking.)

An optical illusion is no less real than anything else. The optical illusion is a structure nested in the workings of brain. Consciousness is the same. It really exists, but that doesn’t mean we can assume it is foundational to any arbitrary external object in Reality. Knowing this, we must also be wary of most panpsychism.

And because we know that much of the reality that we come into contact with is of this optical illusion kind – a phenomenologically real percept but map-wise, a deceiving percept – we must not assume that qualia is simple to capture. For instance, some misguided people suggest a thought experiment which supports eliminativism by suggesting that we can be confused about qualia based on our expectations. It goes like this: A woman is sitting at a researchers office with her back exposed and is told that she will be prodded with a hot utensil. The utensil is actually placed in a freezer and is instead very cold. When it is pressed against her back, the back feels cold qualia but she interprets it as hot qualia, therefore even if qualia exists, we cannot know about it empirically.

This thought experiment is non-sense because the woman felt hot qualia, period. There was no cold qualia regardless of our own outside intuition which knows the utensil was cold. Consciousness is a creative process that is being hallucinated in the brain, and only sometimes is accurate at representing the environmental inputs. Qualia has nothing to do with externalities which are not part of the internal simulation. So as long as we can understand the topology, information-processing events, molecular snapshots, and so on, a map corresponding to the internal simulation can be built, and the good appearances identified for future precision targeting.




On Reaching Utopia

Now that we can pause to describe a method for monitoring and replanning, we need to ask, “Can Utopia work?”  This is a surprisingly tricky question. If we mean “Can we guarantee that sentience will always achieve Utopia?” then the answer is no, because the sentience could inadvertently arrive at a dead end from which there is no repair. For example, the human sentience might have faulty models of herself and not know that her existence can rot away along with her body. Once the body does, it cannot repair any plans to reach Utopia. If we rule out dead ends–assume that there exists a plan to reach the goal from any state in the environment–and assume that the environment is really nondeterministic, in the sense that a plan always has some chance of success on any given execution attempt, then the sentience will eventually reach Utopia.

Trouble occurs when an action is actually not nondeterministic, but rather depends on some precondition that the sentience does not know about. For example, sometimes a mind configuration may be unsatisfactory, so becoming isomorphic to that configuration has no effect towards Utopia. No amount of retrying is going to change this. One solution is to choose randomly from among the set of possible reconfiguration plans, rather than to try the same one each time. In this case, the reconfiguration plan of becoming another mind might work. A better approach is to learn a better model. Every prediction failure is an opportunity for learning; a sentience should be able to modify its model of the world to accord with its percepts. From then on, the replanner will be able to come up with a reconfiguration that gets at the root problem, rather than relying on luck to choose a good reconfiguration.



Raspberry-candy nebulas crack. Angels gush, velvet and electric. There are no forces, only fields that glow dense like her spicy tongue in your mouth. The wicks inside burn, it hurts when they burn – black char to void. The asteroids are here for your entertainment. Break them! Break them all!

Eternal Block Time



Event B is simultaneous with A in the green reference frame, but it occurred before in the blue frame, and will occur later in the red frame.

From Wikipedia:

Special relativity suggests that the concept of simultaneity is not universal: according to the relativity of simultaneity, observers in different frames of reference can have different measurements of whether a given pair of events happened at the same time or at different times, with there being no physical basis for preferring one frame’s judgments over another’s. However, there are events that may be non-simultaneous in all frames of reference: when one event is within the light cone of another—its causal past or causal future—then observers in all frames of reference show that one event preceded the other. The causal past and causal future are consistent within all frames of reference, but any other time is “elsewhere”, and within it there is no present, past, or future. There is no physical basis for a set of events that represents the present.[8]

If you get this, you can move on. If not, then let me try to explain this simply; with analogies and without jargon.

We can be separated by space. For example, I can be standing on the sand at the shore and you can be knee-deep in the sea. Say we agree to raise our hand up at the same time, and do so. It may seem that this event proves there is a now. There is a snapshot of time in which we both raise our hand up.

However, there exist ways for a third observer to see me raising my hand first, or you raising your hand first. This has nothing to do with the biomechanics of reaction speeds or cognitive illusions. It is possible for an observer to literally see and measure what are precisely two simultaneous events from our perspective to not be simultaneous. This is because we all exist on our own “reference frame.”

It is impossible to affirm that I universally raised my hand at the same time as you. We did or didn’t, both views can be physically correct.

For example, a princess falls asleep in Tokyo and another on the Moon. Imagine a being halfway between these two places that has godlike vision, it sees them give their last blink at the same time. However, if another being is flying from Tokyo towards the Moon, it will see the princess on the Moon doze off first. It does not make sense to ask, “But which one really happened?” The god-eyed being resting between Tokyo and the Moon could take a photo of the situation, and then later meet up to compare this with the photo taken by the other god-eyed being who had been soaring to the Moon, and they would have different photos. If they then compare their results with yet another frozen snapshot taken by a being who had been plunging from the Moon towards Tokyo, they would find evidence of another version of the events in which the Tokyo princess was already asleep while the other’s unmistakable aquamarine gaze was yet peering into the stars.

There is a way for the universe to line up the events so that all reference frames agree that one of the princesses fell asleep first. The universe does this by gathering up the fragments and connecting them on a strand of light. This is called causality, and this is how it’s done:

The Tokyo princess closes her eyes. Now quick, count, 1 tick, 2 tock, 3 tick, 4 tock, 5 tick, 6 tock, 7 tick, 8 tock, 9 tick, and the princess on the Moon closes her eyes. We have time 9 seconds. Now set aside this 9 we have collected for we will need to weigh it against another number. If the 9 we have collected is greater than the number we will collect, then we will succeed at preserving the order.

Now we must create the opponent. To transmute 9’s contender, we must take the distance 238,900 mi (from Tokyo to Moon) and divide by 186,282 miles per second (the speed of light)… And the opponent created from distance and light speed, measures in at 1.28.

Now weigh these and pray that our 9 is larger than this 1.28. Yes! 1.28 is definitely smaller than 9. We have succeeded at preserving the order! Now no one will have to disagree that the princess in Tokyo closed her eyes first.

Notice that the universe only succeeded because it didn’t see the princesses doze off at the same time. But what if it saw 0 time elapse between the shutting of each of the lady’s respective eyelids? This is what the god-eyed being resting half-way between the Moon and Tokyo saw. It is not what the being shooting head-first toward the Moon saw, and this is unavoidable. There are different reference frames. Shakespeare said, “All the world’s a stage.” He was wrong. You are wrapped in your stage as you move through the world.

This means that your past can be in someone else’s future, and your future can be another’s past, so long as you are not causally linked.

Favorite Bands


The Firstborn

Shape 7

The Firstborn is a Portuguese band that does Buddhist metal. Enough said.

No but really, they capture the radical, dark beauty of both Tibetan and early Buddhism. There is, in the eastern Christ, a hard to express combination of Thanatos(death drive), with hardcore realism, and a vast network of action plans for self-administration of wireheading. At one moment the Buddha says it is better to swallow a sphere of hot iron than to lay in bed with a woman, the next, he is advocating practicing compassion for all beings. Within the framework of rebirth, there is no contradiction. His obsession with eliminating suffering is haunting and refreshingly bleak. I’ve always admired that. And this band seems to get it.



Shape 6

I know it’s weird that I’m recommending religious bands, but here goes an Evangelical Christian band: Thrice.

Their theology leads to a fucked-up-world-view. But they capture the stark sentimentality of Christianity with genuinely intelligent poetic skill.

“Your eyes resting in flame
Leaving me breathless again
Like hydrogen split on fault lines”


The Ocean


Luckily, there is a band that proselytizes the truth of evolution, and the error of heliocentrism. When I meet Richard Dawkins, I will tell him about this band.




For those who have upgraded their concerns from common-sense atheism/humanism to transhumanism.

Featuring lyrics lamenting our arbitrary human condition such as:

“I am a product of chaos and entropy; as the dusk’s kicking in, I get tired… …Reborn in a cosmic fire, will I die or return from demise? Is it already over? Am I falling apart? Out in the dark and into a fading white star.”

And the techno-optimism often fervent in these circles:

Applying Lessons From AI To Our Behavior

Maybe you’ve heard about Sam Harris’s analogy of the “moral landscape” in which Harris urges us to think about morality in terms of human and animal well-being, viewing the experiences of conscious creatures as peaks and valleys on a “moral landscape.”

Now, in artificial intelligence, there is something called a hill-climbing search. –You must now see the obvious link here.–

However there are several problems with hill-climbing search in its simplest form. First of all, hill-climbing search keeps just one current state in memory. Unfortunately, it leaves the agent sitting at local maxima with nowhere to go.

This will be an extremely relevant problem if we took it as our mission to find a peak in the moral landscape. Humans can be very much like agents that keep just one current state in memory.

Imagine we stumbled upon a post-scarcity society on Earth where everyone is plugged into a highly pleasurable virtual reality of their own liking. Is it reasonable to assume that anyone will want to wake from the dream and search for something more? Probably not. There is nothing in their immediate perception that encourages them to stop what they are doing.

This is the problem that Nietzsche identified with the Utilitarians. He suspected that such philosophies would encourage no effort to produce something higher. And he suggested that there are higher peaks that can only be achieved while incurring great suffering.

A philosophy that only looks at the now is bound to hit a local maxima and stay there. This is a problem with Buddhism. It is true that we would all be happier if we could all renounce the world and meditate in community. That is absolutely true. I am convinced it is not a scam.

However, this would still be a horrible outcome for humanity and our descendants. It would mean that there would be no more feverish technological progress catalyzed by Asperger-y, neurotic people. There would be no competition, and pressure to push the boundaries of medicine and science.

It sounds good from the point of view of “now” but from the point of view of the “big picture” it would mean we never cure aging, become integrated with an expanding galactic God, transcend our flesh to explore the vast realm of creativity and selfless joys in the virtual datascape, and so on. Our descendants would miss out on things we never knew existed.

Moreover, a problem with the hill-climbing algorithm is that random restarts cannot be used, because the agent cannot transport itself to a new state. This is also an importable analogy to describe our situation if we don’t make sure to hold tightly to a drive that creates new knowledge. We need to be placed in new environments naked against the strange, cold, winds of the unknown. This causes us to suffer – or at minimum, takes resources away from what could be producing good qualia – but if we cannot cast ourselves in a leap of faith from our peak, then we will never know just what we missed.

That applies even to the peaceful, thriving, post-scarcity economy in full-immersion realities. They should not stay there, and say “good enough.” They should send some randomizing probes to explore new configurations, until they stumble upon a higher peak, and then again. These probes would probably need to be conscious in order to report back on their newly charted territory. So they would necessarily be martyrs for the greater good in some way. The only way to avoid this Genesis-on-loop scenario is to have a fully developed science of consciousness, so that the peaks of experience can be specified physically down to atomic configuration without having to send already-sentient minds bouncing around to find them. Here is some work beginning an approach to a formal science of qualia.

A somewhat childish thought that I’ve had for a while is that if we take Nick Bostrom’s simulation argument seriously and thus assign some significant probability that we are nested several layers deep in the matrix, then it is easy to view us as doing a random walk to explore the environment. We have many copies, each trying out different actions at the quantum level, but over time, these accumulate to noticeable differences. The being(s) outside the simulation may be looking for a solution, mapping the qualia landscape with us. Not good reasoning, but good for theologians rapidly losing stock value on their 1st century desert-aesthetic. Feel free to take that idea.

Now, what are the more practical lessons that we can derive and use today based on these observations?

  1. Do random stuff every so often. Learn random stuff. Randomize a Wikipedia article until something valuable comes up. It could change your life.
  2. Don’t worry too much about hedonic calculations. If you feel like puking while running, sometimes you just have to say “fuck it” and keep running.



The Fabric of Spacetime vs. Utilitarianism

The sensation of a speck of dust in the eye cannot be packed into cubits of suffering and amassed to outweigh hellish torture. Given the option of having a universe of 5 minutes of hellish torture or a universe that contains a speck of dust in the eye repeating on loop for 300000000000000000000 gazillion years to the power of Graham’s number, I would take the latter. That experience does not including boredom or anything else we might associate with living a long time: there is no memory, no complicated sense of me, just that isolated grain of slightly unpleasant experience on repeat. This is clearly the better option if one understands that pains cannot be reduced to basic units and then added with basic integers. Experiences must instead be considered akin to intricate structures, like the geometries of molecules composed of atoms.

This leads to serious conclusions that are perhaps depressing to some:

For the problem of finding a route from Arad to Bucharest, the search cost is the amount of time taken by the search and the solution cost is the total length of the path in kilometers. For the problem of finding a route from shrew-like mammal to planet-sized upload, the search cost is tens of millions of years and the solution cost is instances of torture and suboptimal qualia miscellanea that occurred throughout life’s history. Thus to compute the cost, we cannot add milliseconds to kilometers like in Arad to Bucharest –we would have to add tens of millions of years to “unforgivable integers.” Since the operation cannot be done, utilitarianism, as naively formulated, necessarily fails.

Some may dispute that there exist classes of sufferings so bad as to require unforgivable integers. A portion of the people who hold this view are making the dust speck fallacy. But we do not bleed cubits of infinitesimal hurt, we bleed complex structures in mindspace. The applications of calculus don’t work for phenomenology. The infamous little epsilon as the limit approaches infinity just doesn’t exist here in the physical state space identical to the qualia in question.

Whether we take an IIT view of consciousness or just a standard neuroscience mapping, there is no place for utilitarian reductionism of the contents of experience. The contents of experience in each subjective “slice of now” are determined by complex physical arrangements. If you wanted to reduce a “now slice” of valences derived from sensations, and sights, and sounds, and moods, and thoughts into a single value, this point would have to exist on a ridiculously multidimensional graph. And while I do think there is a suffering-to-thriving axis hiding there in a complex way, we cannot yet find it, much less perform additions.

I, for one, as witness to the tragic suffering of a single girl, would rather save her from this misery than perform any calculation. Could I have been there to set flames to the primordial soup in order to prevent the torture of a single girl, I would have done it… and then proceeded to anesthetize the stars, forever revoking the words “let there be light.”

I hold this unpopular view especially because I’m not a solipsist with regard to the direction of time, as most people are. Most people believe I exist just as much as they exist – that there is a frame in my inner movie just as there is a movie frame in the other person’s skull across the room, so long as we are only separated by distance in space. But they refuse to believe that someone in the past is experiencing. They believe that the past is deleted as the big platform of Now pushes everyone forward.

However, we do not exist in a Newtonian universe. Our universe is better described by Einstein’s theory of relativity. This means that past people exist just as much as present people and future people. There is no big now to which we all belong to in a room. And this is just a fact that has to be grappled with – the theory of relativity is not up for debate. We are not all riding on the same platform. 

As we move, we bend the fabric of spacetime, changing the rates of our clock-ticks but never cheating our age. This produces a scramble out of our neatly packed intuitions of “past beings” and “future beings.” And it is especially noticeable at large distances and/or speeds based on who’s observer’s present we are dealing with. It’s a mess to wrap our heads around, but this is no reason to comfort ourselves in the delusion that physics is stuck in the early 18th century.

So given that torture is ever-present, even if paradise engineering was possible – where all matter is reconfigured into pure pleasure, happiness, and awe in a shockwave of benevolent nanotech spreading across the galaxies – I believe that it would be more morally appropriate to just shut off the universe.

If you still disagree, I guess the debate boils down to a Buddhist vs. a Christian, even if we are both atheists.

The Buddhist sees suffering, aging, and death, drops everything and develops an exit plan.

The Abrahamist sees suffering and convinces herself that the recompense is worth it in the end, no matter who else is damned.





Consensus Reality Is Dead

Zip up your pants 21st Century, you’ve pissed on consensus reality’s grave enough now.

That’s right. The illusion of safety: The Bible, Shakespeare, Star Trek. This is over. The filters that once provided a common lens to shield us from the Infinite, from the Great Arbitrariness, they lay broken at our feet. Our delicate irises now expose their yielding flesh to the hard light we have unraveled.

We see in textbooks the final chance to pimp the truth. The thousand page punctures read like the dying ticks of greedy claws.

Forty years from now, there will be no pop culture. You will not be able to simply pick out references that everyone will understand. Abraham Lincoln – that was just some human like Konrad Weichert was some human. The Holocaust – that was just some genocide, like the destruction of Melos by the Athenians.

The once successful memes will be flushed out of our brains as they face increasing competition. A reckless flood of information is what we have plunged into. The roaring rapids of the internet churn out new ideas without rest, they beep away like digital bacteria evolving in our screens.

Socially-enforced dogmas – these structures of common ground – cannot grow without attention. Once they are depleted of that precious limited resource, they will perish.

In this climate, the contents of our minds will increasingly contract from Cultural Cannon [Trump, Youtube Front Page, Google News], to Tribal [Breitbart, LessWrong, <insert your favorite public person(s) to stalk>] , to Personalized [your own revealed intellectual interests, aesthetic senses, emotional tendencies, attention span, IQ, are all factored in by big data and machine learning algorithms to feed you content that will maximally entertain you].

But as we head to bury ourselves in our randomly allotted corner of the pantheon, can we pause and ask for the truth? The capital “t” kind of truth? Will Wikipedia provide it? ––Something like it may one day turn out to be our last bastion of dispassionate reason, and I don’t say that sarcastically. But it doesn’t provide a direction.

To put it bluntly, the direction needs to be provided by a bully – by an alpha male. We are apes that evolved for the larger part of our history in a savanna. The Catholic Church, the Abbasid Caliphate, the Soviet Union, the United States of America, these have all been holographic tribe leaders towering over our heads. But they are no more. Nations will die, religions will die. We are living at the bleeding edge of the final era to witness such creatures. Well served. Rest in peace, bastards.

Now we proceed with our own two feet. We will not be given the option to have meta-guidance even if we want it, even if we plead and beg, there will be no-one filling the role.

The education system, once a prime, fit, hologram alpha. This brick organism, by means of its army of underachiever adult appendages once had sufficient power to round up and imprison young homo sapiens. Now it crumbles with cyber-grenades and free-video mortars. The war drones are the iPhones and the laptops shooting endless rounds of business opportunities and fun distractions. Google will never run out of ammunition. At this rate, physical schools cannot survive. There is no special knowledge hiding behind those walls. They have been exposed.

Almost everything I know that is of importance to me, I have learned outside of the classroom curated experience. An understanding of the evolutionary reason for emotions, learning about stocks, getting a sense of big history, getting a sense of what this cosmos is, understanding the possibilities for the future in the universe, contemplating what is morality and consciousness, even the very notion that all is governed by natural law – none of these investigations and realizations were precipitated in school. They came through self-inquiry against the mirror of a search engine.*

So in the midst of this revolution, someone must rise to the challenge. There needs to be an evangelist soldier to raise the empire of truth for the new brains infected with bandwidth anxiety and tremendous freedom.

At sixteen, I wrote a business plan detailing my idea for a church of science. I never pitched it to anyone. And I’m now quite more agnostic about the value of such a thing IRL. The Truth must be preserved aesthetically, molded precisely to the cognitive constitution of the individual. And the only way to enforce the Truth is through voluntary self-conscription. If we at all value Truth, we must begin to specify what it is (…probably not a set of words, but a path) and preparing that which it is for our own future consumption, lest we wander aimlessly past the event horizon of tailored content, never to return.








*If I didn’t have the internet, I would never have heard of quantum mechanics, much less been able to peep into discussions about P vs. NP, machine learning, transhumanism, qualia, and dark energy. Nope. Not in my side of town. I grew up with uneducated immigrant parents. They settled in a lower-middle income city, and I was exposed to a ghetto culture in school.

The great equalizer is not public education. If you end up in a rich suburb, you tend to stay well-off, and if you are bred into an anti-intellectual, entrepreneurially scarce environment, it’s hard to spontaneously rip your environment a new one. This inability is partially due to the geographical distribution of genetic traits, so you are more likely to be less smart or driven if your parents were the type to end up in a slum. This is a gruesome statistical fact that makes many people flinch. Nowadays, most publicly-visible “privileged” people flinch on this matter for well-intentioned reasons, but I believe it is important to acknowledge it. Not from a place of condescending privilege, but from a place of seeing that we have all drawn the short end of the stick; some worse than others, but germ-line genetic engineering and neuralink are just around the corner. We need to start thinking now about how these technologies should benefit everyone and not allow ourselves to create an insurmountable ultra-elite.




Seedless Plants

Colonization of land by plants fundamentally altered the history of life on Earth. A terrestrial environment offers abundant CO2 and solar radiation for photosynthesis. But for at least 500 million years, the lack of water and higher ultraviolet (UV) radiation on land confined green algal ancestors to an aquatic environment. Evolutionary innovations for reproduction, structural support, and prevention of water loss are key in the story of plant adaptation to land. The evolutionary shift on land to life cycles dominated by a diploid generation masks recessive mutations arising from higher UV exposure. As a result, larger numbers of alleles persist in the gene pool, creating greater genetic diversity. Long before seeds and flowers evolved, the seedless plants covered the Earth. In this chapter we consider the evolutionary innovations in seedless plants during the first 100 million years of terrestrial life.

Origin of Land Plants

Green algae and the land plants shared a common ancestor a little over 1 BYA and are collectively referred to as the green plants. DNA sequence data are consistent with the claim that a single individual gave rise to all green plants. The green plants are photoautotrophic, but not all photoautotrophs are plants. The definition of a green plant is broad, but it excludes the red and brown algae. All algae–red, brown, and green–shared a primary endosymbiotic event 1.5 BYA. But sharing an ancestral chloroplast lineage is not the same as being monophyletic, as discussed in chapter 29. Red and green algae last shared a common ancestor about 1.4 BYA. Brown algae became photosynthetic through endosymbiosis with a eukaryotic red alga that had itself already acquired a photosynthetic cyanobacterium, as described in the preceding chapter.
Plants are also not fungi, which are more closely related to metazoan animals. Fungi, however, were essential to the colonization of land by plants, enhancing plants’ nutrient uptake from the soil.
One of the most significant evolutionary events in the billion-year-old history of the green plants is the adaptation to terrestrial living.

Land plants evolved from freshwater algae

Some saltwater algae evolved to thrive in a freshwater environment. Just a single species of freshwater green algae gave rise to the entire terrestrial plant lineage, from mosses through the flowering plants (angiosperms). Given the incredibly harsh conditions of life on land, it is not surprising that all land plants share a single common ancestor. Exactly what this ancestral alga was is still a mystery, but close relatives, members of the charophytes, exist in freshwater lakes today.
The green algae split into two major clades: the chlorophytes, which never made it to land, and the charophytes, which are sister clade to all land plants. Together charophytes and land plants are referred to as streptophytes. Land plants, although diverse, have certain characteristics in common. Unlike the charophytes, land plants have multicellular haploid and diploid stages. Diploid embryos are also land plant innovations. Over time, the trend has been toward more embryo protection and a smaller haploid stage in the life cycle.

Land plants have adapted to terrestrial life

Unlike their freshwater ancestors, most land plants have only limited amounts of water available to them. As an adaptation to living on land, most plants are protected from desiccation–the tendency of organisms to lose water to the air–by a waxy surface material called the cuticle that is secreted onto their exposed surfaces. The cuticle is relatively impermeable, preventing water loss. This solution, however, limits the gas exchange essential for respiration and photosynthesis. Gas diffusion into and out of a plant occurs through tiny mouth-shaped openings called stomata (singular, stoma), which allow water to diffuse out at the same time. Stomata can be closed at times to limit water loss.
Moving water within plants is a challenge that increases with plant size. Members of the land plants can be distinguished based on the presence or absence of tracheids, specialized cells that facilitate the transport of water and minerals. Tracheophytes have specialized transport cells called tracheids and have evolved highly efficient transport systems: water-conducting xylem and food-conducting phloem strands of tissue in their stems, roots, and leaves. Some plants that grow in aquatic environments, including water lilies, have tracheids. Aquatic tracheophytes had terrestrial ancestors that adapted back to a watery environment.
Terrestrial plants are exposed to higher intensities of UV irradiation than aquatic algae, increasing the chance of mutation. Diploid genomes mask the effect of a single, deleterious allele. All land plants have both haploid and diploid generations, and the evolutionary shift toward a dominant diploid generation allows for greater genetic variability to persist in terrestrial plants.

The haplodiplontic cycle produces alternation of generations

Humans have a diplontic life cycle, meaning that only the diploid stage is multicellular; by contrast, the land plant life cycle is haplodiplontic, having multicellular haploid and diploid stages. Most multicellular green plants have this haplodiplontic life cycle. Many multicellular green algae and all land plants have haplodiplontic life cycles and undergo mitosis after both gamete fusion and meiosis. The result is a multicellular haploid individual and a multicellular diploid individual–unlike in the human life cycle, in which gamete fusion directly follows meiosis.
Many brown, red, and green algae are also haplodiplontic. Humans produce gametes via meiosis, but land plants actually produce gametes by mitosis in a multicellular, haploid individual. The diploid generation, or sporophyte, alternates with the haploid generation, or gametophyte. Sporophyte means “spore plant,” and gametophyte means “gamete plant.” These terms indicate the kinds of reproductive cells the respective generations produce.
The diploid sporophyte produces haploid spores (not gametes) by meiosis. Meiosis takes place in structures called sporangia, where diploid spore mother cells (sporocytes) undergo meiosis, each producing four haploid spores. Spores are the first cells of the gametophyte generation. Spores divide by mitosis, producing a multicellular, haploid gametophyte.
The haploid gametophyte is the source of gametes. When the gametes fuse, the zygote they form is diploid and is the first cell of the next sporophyte generation. The zygote grows into a diploid sporophyte by mitosis and produces sporangia in which meiosis ultimately occurs.

The relative sizes of haploid and diploid generations vary

All land plants are haplodiplontic; however, the haploid generation consumes a much larger portion of the life cycle in mosses and ferns than it does in the seed plants–the gymnosperms and angiosperms. In mosses, liverworts, and ferns, the gametophyte is photosynthetic and free living. When you look at mosses, what you see is largely gametophyte tissue; the sporophytes are usually smaller, brownish or yellowish structures attached to the tissues of the gametophyte. In other plants, the gametophyte is usually nutritionally dependent on the sporophyte. When you look at a gymnosperm or angiosperm, such as most trees, the largest, most viable portion is a sporophyte.
Although the sporophyte generation can get very large, the size of the gametophyte is limited in all plants. The gametophyte generation of mosses produces gametes at its tips. The egg is stationary, and sperm lands near the egg in a droplet of water. If the moss were the height of a sequoia, not only would vascular tissue be needed for conduction and support, but the sperm would have to swim up the tree! In contrast, the small gametophyte of the fern develops on the forest floor where gametes can meet. Tree ferns are especially abundant in Australia; the haploid spores that the sporophyte trees produce fall to the ground and develop into gametophytes.
Having completed an overview of plant life cycles, we next consider the major groups of seedless land plants. As we proceed, you will see a reduction of the gametophyte from group to group, a loss of multicellular gametangia (structures in which gametes are produced), and increasing specialization for life on land.

All algae acquired chloroplasts necessary for photosynthesis, but green algae diverged from red algae after that event. A single freshwater green alga successfully invaded land; its descendant eventually developed reproductive strategies, conducting systems, stomata, and cuticles as adaptations. Green plants include all green algae and the land plants, whereas the streptophytes include only the land plants and their sister clade, the charophytes. Most plants have a haplodiplontic life cycle, a haploid form alternates with a diploid form in a single organism. Diploid sporophytes produce haploid spores by meiosis. Each spore can develop into a haploid gametophyte by mitosis; the gametophyte form produces haploid gametes, again by mitosis. When the gametes fuse, the diploid sporophyte is formed once more.

Bryophytes: Dominant Gametophyte Generation

Land plants began diverging 450 MYA. Bryophytes are the closest living descendants of these first land plants. Plants in this group are also called nontracheophytes because they lack the derived transport cell called a tracheid.
Fossil evidence and molecular systematics can be used to reconstruct early terrestrial plant life. Water and gas availability were limiting factors. These plants likely had little ability to regulate internal water levels and likely tolerated desiccation, traits found in most extant mosses, although some are aquatic.
Algae, including the Charales, lack roots. Fungi and early land plants cohabited, and the fungi formed close associations with the plants that enhanced water uptake. The tight symbiotic relationship between fungi and plants, called mycorrhizal associations, are also found in many existing bryophytes.

Bryophytes are unspecialized but successful in many environments

The approximately 24,700 species of bryophytes are simple but highly adapted to a diversity of terrestrial environments, even deserts. Most bryophytes are small; few exceed 7 cm in height. Bryophytes have conducting cells other than tracheids for water and nutrients. The tracheid is a derived trait that characterizes the tracheophytes, all land plants but the bryophytes. Bryophytes are sometimes called nonvascular plants, but nontracheophyte is a more accurate term because they do have conducting cells of different types.
Scientists now agree that bryophytes consist of three quite distinct clades of relatively unspecialized plants: liverworts, mosses, and hornworts. Their gametophytes are photosynthetic and are more conspicuous than the sporophytes. Sporophytes are attached to the gametophytes and depend on them nutritionally in varying degrees. Some of the sporophytes are completely enclosed within gametophyte tissue; others are not and usually turn brownish or straw-colored at maturity. Like ferns and certain other vascular (tracheophyte) plants, bryophytes require water (such as rainwater) to reproduce sexually, tracing back to their aquatic origins. It is not surprising that they are especially common in moist places, both in the tropics and temperate regions.

Liverworts are an ancient phylum

The Old English word wyrt means “plant” or “herb.” Some common liverworts (phylum Hepaticophyta) have flattened gametophytes with lobes resembling those of liver–hence the name “liverwort.” Although the lobed liverworts are the best known representatives of this phylum, they constitute only about 20% of the species. The other 80% are leafy and superficially resemble mosses. The gametophytes are prostrate instead of erect, with single celled rhizoids that aid in absorption like roots but are not organs.
Some liverworts have air chambers containing upright, branching rows of photosynthetic cells, each chamber having a pore at the top to facilitate gas exchange. Unlike stomata, the pores are fixed open and cannot close.
Sexual reproduction in liverworts is similar to that in mosses. Lobed liverworts may form gametangia in umbrella-like structures. Asexual reproduction occurs when lens-shaped pieces of tissue that are released from the gametophyte grow to form new gametophytes.

Mosses have rhizoids and water-conducting tissue

Unlike other bryophytes, the gametophytes of mosses typically consist of small, leaflike structures (not true leaves, which contain vascular tissue) arranged spirally or alternately around a stemlike axis; the axis is anchored to its substrate by means of rhizoids. Each rhizoid consists of several cells that absorb water, but not nearly the volume of water that is absorbed by a vascular plant root.
Moss leaflike structures have little in common with leaves of vascular plants, except for the superficial appearance of the green, flattened blade and slightly thickened midrib that runs lengthwise down the middle. Only one cell layer thick (except at the midrib), they lack vascular strands and stomata, and all the cells are haploid. However, mosses do have stomata on the capsule portion of the sporophyte generation and because of that are the basal land group with stomata.
Water may rise up a strand of specialized cells in the center of a moss gametophyte axis. Some mosses also have specialized food-conducting cells surrounding those that conduct water.

Moss reproduction

Multicellular gametangia are formed at the tips of the leafy gametophytes. Female gametangia (archegonia) may develop either on the same gametophyte as the male gametangia (antheridia) or on separate plants. A single egg is produced in the swollen lower part of an archegonium, whereas numerous sperm are produced in an antheridium.
When sperm are released from an antheridium, they swim with the aid of flagella through a film of dew or rainwater to the archegonia. One sperm (which is haploid) unites with an egg (also haploid), forming a diploid zygote. The zygote divides by mitosis and develops into the sporophyte, a slender, basal stalk with a swollen capsule, the sporangium, at its tip. As the sporophyte develops, its base is embedded in gametophyte tissue, its nutritional source.
The sporangium is often cylindrical or club-shaped. Spore mother cells within the sporangium undergo meiosis, each producing four haploid spores. In many mosses at maturity, the top of the sporangium pops off, and the spores are released. A spore that lands in a suitable damp location may germinate and grow, using mitosis, into a threadlike structure, which branches to form rhizoids and “buds” that grow upright. Each bud develops into a new gametophyte plant consisting of a leafy axis.

Moss distribution

In the Arctic and the Antarctic, mosses are the most abundant plants. The greatest diversity of moss species, however, is found in the tropics. Many mosses are able to withstand prolonged periods of drought, although mosses are not common in deserts.
Most mosses are highly sensitive to air pollution and are rarely found in abundance in or near cities or other areas with high levels of air pollution. Some mosses, such as the peat mosses (Sphagnum), can absorb up to 25 times their weight in water and are valuable commercially as a soil conditioner or as a fuel when dry.

The moss genome

Moss plants can survive extreme water loss–an adaptive trait in the early colonization of land that has been lost from vegetative tissues of tracheophytes. Desiccation tolerance and phylogenetic position were among the traits that led researchers to sequence the genome of the moss Physcomitrella patens as being the first land plant that is not a tracheophytes. Although the moss genome is a single genome bracketed by Chlamydomonas and the tracheophytes, many evolutionary hints are hidden within it. Evidence indicates the loss of genes associated with a watery life, including flagellar arms, were lost in the last common ancestor of the land plants. Genes associated with tolerance of terrestrial stresses, including temperature and water availability, are absent in Chlamydomonas and present in moss. For example, the plant hormone abscisic acid (ABA) is important in stress responses in moss and other land plants and genes needed for ABA signaling are not found in algae.

Hornworts developed stomata

The origin of hornworts (phylum Anthocerotophyta) is a puzzle. They are most likely among the earliest land plants, yet the earliest hornwort fossil spores date from the Cretaceous period (65-145 MYA), when angiosperms were emerging.
The small hornwort sporophytes resemble tiny green broom handles or horns, rising from filmy gametophytes usually less than 2 cm in diameter. The sporophyte base is embedded in gametophyte tissue, from which it derives some of its nutrition. However, the sporophyte has stomata to regulate gas exchange, is photosynthetic, and provides much of the energy needed for growth and reproduction. Hornwort cells usually have a single large chloroplast.

The bryophytes exhibit adaptations to terrestrial life. Moss adaptations include rhizoids to anchor the moss body and to absorb water, and water-conducting tissues. Mosses are found in a variety of habitats, and some can survive droughts. Hornworts developed stomata that can open and close to regulate gas exchange.

Tracheophyte Plants: Roots, Stems, and Leaves

Tracheophytes, also known as vascular plants, first appeared about 410 MYA. The first tracheophytes with a relatively complete record belonged to the phylum Rhyniophyta. We are not certain what the earliest of these vascular plants looked like, but fossils of Cooksonia provide some insight into their characteristics.
Cooksonia, the first known vascular land plant, appeared in the late Silurian period about 420 MYA, but is now extinct. It was successful partly because it encountered little competition as it spread out over vast tracts of land. The plants were only a few centimeters tall and had no roots or leaves. They consisted of little more than a branching axis, the branches forking evenly and expanding slightly toward the tips. They were homosporous (producing only one type of spore). Sporangia formed at branch tips. Other ancient vascular plants that followed evolved more complex arrangement of sporangia.

Vascular tissue allows for distribution of nutrients

Cooksonia and the other early plants that followed it became successful colonizers of the land by developing efficient water water- and food-conducting systems called vascular tissues. These tissues consist of strands of specialized cylindrical or elongated cells that form a network throughout a plant, extending from near the tips of the roots, through the stems, and into true leaves, defined by the presence of vascular tissue in the blade. One type of vascular tissue, xylem, conducts water and dissolved minerals upward from the roots; another type of tissue, phloem, conducts sucrose and hormones throughout the plant. Tracheids are the cells in the early vascular plants that conducted water in xylem tissue. Vascular tissue enables enhanced height and size in the tracheophytes. It develops in the sporophyte, but (with a few exceptions) not in the gametophyte. A cuticle and stomata are also characteristic of vascular plants.

Tracheophytes are grouped in three clades

Three clades of vascular plants exist today: (1) lycophytes (club mosses), (2) pterophytes (ferns and their relatives), and (3) seed plants.  Advances in molecular systematics have changed the way we view the evolutionary history of vascular plants. Whisk ferns and horsetails were long believed to be distinct phyla that were transitional between bryophytes and vascular plants. Phylogenetic evidence now shows they are the closest living relatives to ferns, and they are grouped as pterophytes.
Tracheophytes dominate terrestrial habitats everywhere, except for the highest mountains and the tundra. The haplodiplontic life cycle persists, but the gametophyte has been reduced in size relative to the sporophyte during the evolution of tracheophytes. A similar reduction in multicellular gametangia has occurred as well.

Stems evolved prior to roots

Fossils of early vascular plants reveal stems, but no roots or leaves. The earliest vascular plants, including Cooksonia, had transport cells in their stems, but the lack of roots limited the size of these plants.

Roots provide structural support and transport capability

True roots are found only in the tracheophytes. Other, somewhat similar structures enhance either transport or support in nontracheophytes, but only roots have a dual function: providing both transport and support. Lycophytes diverged from other tracheophytes before roots appeared, based on fossil evidence. It appears that roots evolved at least two separate times.

Leaves evolved more than once

Leaves increase surface area of the sporophyte, enhancing photosynthetic capacity. Lycophytes have single vascular strands supporting relatively small leaves called lycophylls. True leaves, called euphylls, are found only in ferns and seed plants, having distinct origins from lycophylls. Lycophylls may have resulted from vascular tissue penetrating small, leafy protuberances on stems. Euphylls most likely arose from branching stems that became webbed with leaf tissue.
About 40 million years separates the appearance of vascular tissue and the wide euphyll leaf–a curiously long time. The current hypothesis is that a 90% drop in atmospheric CO2 360 MYA allowed for the increase in leaf size because of an increase in the number of stomata on a leaf. Large, horizontal leaves capture 200% more radiation than thin, axial leaves. Although beneficial for photosynthesis, larger leaves correspondingly increase leaf temperature, which can be lethal. Stomatal openings in the leaf enhance the movement of water out of the leaf, thereby cooling it. The density of stomata on leaf surfaces correlates with CO2 concentration, as the stomatal openings are essential for gas exchange. As the atmospheric CO2 levels dropped, plants could not obtain sufficient CO2 for photosynthesis. In the low-CO2 atmosphere, natural selection favored plants with higher stomatal densities. Higher stomatal densities favored larger leaves with a photosynthetic advantage that did not overheat. Leaves up to 120 mm wide and 160 mm long have been identified in the fossil record form that time period.

Seeds are another innovation in some tracheophytes phyla

Seeds are highly resistant structures well suited to protecting a plant embryo from drought and to some extent from predators. In addition, almost all seeds contain a supply of food for the young plant. Lycophytes and pterophytes do not have seeds.
Fruits in the flowering plants (angiosperms) add a layer of protection to seeds and have adaptations that assist in seed dispersal, expanding the potential range of the species. Flowers allow plants to secure the benefits of wide out-crossing in promoting genetic diversity, as discussed in chapter 31. Before moving on to the specifics of lycophytes and pterophytes, review the evolutionary history of terrestrial innovations in the land plants.

Most tracheophytes have well-developed vascular tissues, including tracheids, that enable efficient delivery of water and nutrients throughout the organism. They also exhibit specialized roots, stems, leaves, cuticles, and stomata. Many produce seeds, which protect and nourish embryos.

Lycophytes: Dominant Sporophyte Generation and Vascular Tissue

The earliest vascular plants lacked seeds. Members of four phyla of living vascular plants also lack seeds, as do at least three other phyla known only from fossils. As we explore the adaptations of the vascular plants, we focus on both reproductive strategies and the advantages of increasingly complex transport systems.
The lycophytes (club mosses) are relic species of an ancient past when vascular plants first evolved. They are the sister group to all vascular plants. Several genera of club mosses, some of them treelike, became extinct about 270 MYA. Today, club mosses are worldwide in distribution but are most abundant in the tropics and moist temperate regions.
Members of the 12 to 13 genera and about 1150 living species of club mosses superficially resemble true mosses, but once their internal vascular structure and reproductive processes became known, it was clear that they are unrelated to mosses. The sporophyte stage is the dominant (obvious) stage; sporophytes have leafy stems that are seldom more than 30 cm long.
The lycophyte Selaginella moellendorffii is the first seedless vascular plant with a fully sequenced genome. A few clues to the evolution of vascular plants, hidden in the genome, emerged in comparisons with genomes of flowering plants. Genes that play an important role in establishing leaf polarity in flowering plants are not found in Selaginella, indicative of independent origins of leaflike structures in different vascular plant lineages. Genome differences also reflect differences in developmental pathways leading to reproductive maturity in the sporophyte generation in lycophytes and flowering plants.
Comparing predicted proteins in the Chlamydomonas (green alga), Physcomitrella (moss), and Selaginella with 15 angiosperms revealed 3814 gene families that all the green plants share–the essential instructions for building a green plant. About 3000 new genes were acquired in the transition from the single-celled green alga to the multicellular moss, but only 516 genes were added in the transition from nonvascular to vascular plants. This is a first step in sorting out the evolutionary steps that led to the vascular plants.

Lycophytes are basal to all other vascular plants. Although they superficially resemble bryophytes, they contain tracheid-based vascular tissues, and their reproductive cycle is like that of other vascular plants; however, they lack vascularized leaves.

Pterophytes: Ferns and Their Relatives

The phylogenetic relationships among ferns and their near relations is intriguing. A common ancestor gave rise to two clades: One clade diverged to produce a line of ferns and horsetails; the other diverged to yield another line of ferns and whisk ferns–ancient-looking plants.
Whisk ferns and horsetails are close relatives of ferns. Like lycophytes and bryophytes, they all form antheridia and archegonia. Free water is required for the process of fertilization, during which the sperm, which have flagella, swim to and unite with the eggs. In contrast, most seed plants have nonflagellated sperm.

Whisk ferns lost their roots and leaves secondarily

In whisk ferns, which occur in the tropics and subtropics, the sporophytic generation consists merely f evenly forking green stems without roots. The two or three species of the genus Psilotum do, however, have tiny, green, spirally arranged flaps of tissue lacking veins and stomata. Another genus, Tmesipteris, has more leaflike appendages. Currently, systematists believe that whisk ferns lost leaves and roots when they diverged from others in the fern lineage.
Given the simple structure of whisk ferns, it was particularly surprising to discover that they are monophyletic with ferns. The gametophytes of whisk ferns are essentially colorless and are less than 2 mm in diameter, but they can be up to 18 mm long. They form symbiotic associations with fungi, which furnish their nutrients. Some develop elements of vascular tissue and have the distinction of being the only gametophytes known to do so.

Horsetails have jointed stems with brushlike leaves

The 15 living species of horsetails are all homosporous. They constitute a single genus, Equisetum. Fossil forms of Equisetum extend back 300 million years to an era when some of their relatives were treelike. Today, they are widely scattered around the world, mostly in damp places. Some that grow among the coastal redwoods of California may reach a height of 3 m, but most are less than a meter tall.
Horsetails sporophytes consist of ribbed, jointed, photosynthetic stems that arise form branching underground rhizomes with roots at their nodes. A whorl of nonphotosynthetic, scalelike leaves emerges at each node. The hollow stems have silica deposits in the epidermal cells of the ribs, and the interior parts of the stems have two sets of vertical, tubular canals. The larger outer canals, which alternate with the ribs, contain air, and the smaller inner canals opposite the ribs contain water. Horsetails are also called scouring rushes because pioneers of the American West used them to scrub pans.

Ferns have fronds that bear sori

Ferns are the most abundant group of seedless vascular plants, with about 11,000 living species. Recent research indicates that they may be the closest relatives to the seed plants.
The fossil record indicates that ferns originated during the Devonian period about 350 MYA and became abundant and varied in form during the next 50 million years. Their apparent ancestors were established on land as much as 375 MYA. Rainforests and swamps of lycopsid and fern trees growing in the Eastern United States and Europe over 300 MYA formed the coal currently being mined. Today, ferns flourish in a wide range of habitats throughout the world; however, about 75% of the species occur in the tropics.
The conspicuous sporophytes may be less than a centimeter in diameter (as in small aquatic ferns such as Azolla), or more than 24 m tall, with leaves up to 5 m or longer in the tree ferns. The sporophytes and the much smaller gametophytes, which rarely reach 6 mm in diameter, are both photosynthetic.
The fern lifecycle differs from that of a moss primarily in the much greater development, independence, and dominance of the fern’s sporophyte. The fern sporophyte is structurally more complex than the moss sporophyte, having vascular tissue and well-differentiated roots, stems, and leaves. The gametophyte, however, lacks the vascular tissue found in the sporophyte.

Fern morphology

Fern sporophytes, like horsetails, have rhizomes. Leaves, referred to as fronds, usually develop at the tip of the rhizome as tightly rolled-up coils (“fiddleheads”) that unroll and expand. Fiddleheads are considered a delicacy in several cuisines, but some species contain secondary compounds linked to stomach cancer.
Many fronds are highly dissected and feathery, making the ferns that produce them prized as ornamental garden plants. Some ferns, such as Marsilea, have fronds that resemble a four-leaf clover, but Marsilea fronds still begin as coiled fiddleheads. Other ferns produce a mixture of photosynthetic fronds and nonphotosynthetic reproductive fronds that tend to be brownish in color.

Fern reproduction

Ferns produce distinctive sporangia, usually in clusters called sori (singular, sorus), typically on the underside of the fronds. Sori are often protected during their development by a transparent, umbrella-like covering. (At first glance, one might mistake the sori for an infection on the plant.) Diploid spore mother cells in each sporangium undergo meiosis, producing haploid spores.
At maturity, the spores are catapulted form the sporangium by a snapping action, and those that land in suitable damp locations may germinate, producing gametophytes that are often heart-shaped, are only one cell layer thick (except in the center), and have rhizoids that anchor them to their substrate. These rhizoids are not true roots because they lack vascular tissue, but they do aid in transporting water and nutrients from the soil. Flask-shaped archegonia and globular antheridia are produced on either the same or a different gametophyte.
The multicellular archegonia provide some protection for the developing embryo.
The sperm formed in the antheridia have flagella, with which they swim toward the archegonia when water is present, often in response to a chemical signal secreted by the archegonia. One sperm unites with the single egg toward the base of an archegonium, forming a zygote. The zygote then develops into a new sporophyte, completing the lifecycle.
The developing fern embryo has substantially more protection from the environment than a charophyte zygote, but it cannot enter a dormant phase to survive a harsh winter the way a seed plant embryo can. Although extant ferns do not produce seeds, seed fern fossils have been found that date back 365 million years. The seed ferns are not actually pterophytes, but gymnosperms.

Ferns and their relatives have a large and conspicuous sporophyte with vascular tissue. Many have well-differentiated roots, stems, and leaves (fronds). The gametophyte generation is small and lacks vascular tissue.



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