Into the Ocean Without a Paddle - the Mystery of the Red Algae

Most red algae, like this
Batrachospermum, are complex,
multicellular seaweeds (though
this is a freshwater genus).  From
Oltmanns, Morphologie und Biology
der Algen. 1922, Fig.469. 

The term "algae" refers to a wide range of simple aquatic organisms that have chloroplasts.  The red algae are a distinct and natural ("monophyletic") group of algae with reddish pigments in their chloroplasts similar to those of the cyanobacteria.  They are believed to be directly descended from the first eukaryotes to incorporate cyanobacteria as proto-chloroplasts (see my posting "Plants, animals and kleptoplasts, oh my").  The green algae (which later would give rise to the land plants)  have different chloroplast pigments that are an adaptation for life in shallow water.  Though it is not totally settled, DNA evidence suggests that red and green algae are closely related and that the green algae are also directly descended from the first organisms with chloroplasts.  The two groups are believed to have split apart very early on.  The rest of this article assumes that this relationship is true, and explores the mysteries that result from it.


While green algae have varied growth forms, the red algae are mostly complex, multicellular seaweeds.  Seaweeds, like their photosynthetic cousins on land, are organisms that set down "roots" in a suitable location, and stay put for the rest of their life.  But being glued to one spot makes it even more important that they have a means to disperse their genetic information during their reproductive cycles.  In algae, that's typically a 2-step process: spores that travel long distances to mingle with other populations, and sperm cells to travel relatively shorter distances to seek out a suitable egg (see "The truth about sex in plants").  For that they need some means of locomotion, and for single-celled organisms that usually means slender whip-like organelles called flagella (or cilia, which are smaller, but of the same structure). 

They mystery of the red algae arises from the fact that neither their sperm cells nor their dispersal spores have flagella.   Such cells are literally adrift in the ocean "without a paddle."  Throughout this entire group there is no sign of flagella in any species or in any part of the life cycle.  There is no sign even of the flagella-supporting structures (centrioles), usually found in all cells (not just cells with flagella) in other algae and land plants.  It is as if the ancestors of red algae never had flagella at all.  That hypothesis is however complicated by the apparent relationship of red and green algae and their common descent from the first photosynthtic eukaryotes. 

In the unicellular Chlamydomonas, all cells (except for the
dormant zygote) have two flagella.  In multicellular green algae,
only the gametes and zoospores are so equipped. 
From Haupt, Plant Morphology, 1953, Fig. 16.

Flagella are found throughout the eukaryotic domain: in animal and plant sperm cells, in single-celled protists like paramecia and trypanosomes (vectors of sleeping sickness and other diseases), and in the zoospores and sperm cells of most (non-red) algae.  Flagella apparently evolved among the earliest eukaryotes, for their structure is fundamentally the same wherever we find them.  Most green algae produce sperm cells and zoospores with flagella exhibiting that ancient and universal structure.  So the common ancestor of red and green algae must have had flagella.


So the mystery has several layers, including a dilemma: red algae have the older form of chloroplast, and presumably came first, with the green algae evolving from them.  But green algae have the original mode of locomotion provided by flagella, and the flagella-less red algae must have evolved from them!  The simple solution to this is that the first algae had red chloroplasts and flagella, and subsequently split into two groups.  Red algae as we know them today have the original chloroplast but have lost their flagella.  The green algae evolved a new type of chloroplast but retained flagella.

But now we have to ask "why did red algae lose their flagella?"  How do their sperm cells find eggs without any means of self-directed movement?  Textbooks are vague on this issue, generally implying that the loss of flagella must have been accidental.  But it seems that the elimination of flagella and associated support structures, indeed all the genes involvd in producing flagella, was ruthlessly thorough and therefore must have happened for some adaptive "reason."


In red algae such as this Polysiphonia, tiny dust-like sperm cells, called spermatia, are released in great numbers from male plants. As the spermatia swirl around in the water, they are caught by the egg-producing structure on a female plant by long projections called trichogynes. Modified from Raven et. al, Biology of Plants, ed. 7, 2005.

I believe the reason was a change of strategy similar to the shift to wind-pollination in grasses or oaks.  Red algae sperm cells are tiny, barely more than a nucleus.  They are stripped down like this because they don't require the apparatus or energy reserves for locomotion.  They can be produced more cheaply and in much greater numbers.  By releasing huge quantities of tiny sperm cells, red algae are banking on the chance that some of them will randomly drift to the vicinity of egg-producing structures and get caught on them.  This is similar to the grass strategy of producing vast quantities of tiny pollen grains, a few of which get caught in the feathery stigmas of the female pistils.  Grasses exist in large populations in open, windy habitats where this strategy works well.  Presumably, red algae live where similar underwater currents produce the same results.  Green algae, on the other hand, may have evolved in quieter waters, where flagella were still necessary for successful dispersal.

How the grass leaf got its stripes

In monocot leaves, the veins are fundamentally parallel to one
another from the time they enter the leaf base from the stem. 
Some monocot leaves are distorted into other shapes, causing the
veins to bow out from one another.

[For a more complete story of the adaptations and evolution of grasses and other monocots, see Chapter 9 of my book, Plant life - a brief history]

A blade of grass, if you look closely, has long stripes running from its base to its tip.  These are technically the veins containing vascular tissues: xylem and phloem.  These veins conduct water and minerals up into the blade and sugar from photosynthesis down into the stem and roots.  The arrangement is what textbooks call parallel venation.  Each vein enters the broad leaf base independently, and remains parallel and separate from all the others up to the tip of the leaf. 

This anatomical feature of grass leaves speaks to the difference between the traditional division of flowering plants into dicots and monocots (of which grasses are perhaps the epitomy).  Though dicots are no longer recognized as a single distinct clade, they do exhibit a suite of characters by which they differ sharply from the monocots, and therefore can be referred to informally.  

Typical dicot leaves, for example, have a net venation, in which a few veins enter the leafblade via a narrow petiole and then branch repeatedly into a netlike pattern. 

The leaves of dicotyledonous trees, shrubs and herbs have a
 netted pattern of venation developing from the branching
of one or a few primary veins. 
From Kerner & Oliver.  The natural history of plants.  Gresham. London. 1904. 

These contrasting leaf types are the result of a different pattern of growth.  Dicot leaves form as miniatures, with just a few veins, then expand in all directions, adding finer veins between the main veins as they go.  Monocot leaves, on the other hand, produce new leaf tissue at the base, which pushes the older part of the leaf upward.  The many parallel veins appear when the leaf is young and very short, then new tissues are added to the bottom of each as the leaf gets longer.

The leaves of this Liquidambar tree, a dicot,
form in miniature in the terminal bud, then
expand rapidly.  The net-like pattern of
venation developes as ever finer veins are
added between the earlier veins.

The distinctive pattern of growth of the grass leaf is found throughout the monocots, at least in their seedlings, and it appers to have begun in the ancestral monocot as an adaptation to growth from an underground stem.  The tips of the monocot leaves become mature and stiff when they are very short, so as to be able to push through the soil, while the softer, growing bases are nestled below ground among the bases of older leaves.

Many monocots retain this subterranean existence with their primary stems taking the form of rhizomes, bulbs, and corms.  Those that don't, such as single-stemmed palm trees, still have seedling leaves that push up from below the ground.  

The leaves of most dicots, on the other hand, are adapted to form at the tips of exposed twigs, with the miniature versions forming within buds, then expanding rapidly to full size.

The Amaryllis plant consists of an underground bulb, with new leaves arising from the central bud.  One can demonstrate the basal growth of the leaves by making a mark near the base of a new leaf.  The mark rises as new tissues are produced below it.

Seedlings of dicots ( a and b) develop leafy shoots above ground, with the terminal bud and new leaves exposed to the elements, while the seedlings of monocots (c and upper right inset) have highly compressed shoots that keep the terminal bud below ground.  Leaves arising from the buds then push up through the soil, adding new tissues only at the base.

Keeping the main stems, buds and leaf bases below ground is advantageous in variety of conditions.  Plants can survive winters, dry seasons, fires, and grazing by remaining dormant below ground.  This is where grasses excel, giving them mastery over plains and savannas.  The tops of the grass blades can be chewed to the ground, and quickly restored by the basal growth.  But it also serves semi-aquatic plants well, by allowing leaves and shoots to rise above the water level through basal growth.

A number of dicots, such as
this aquatic pennywort, have
petioles that can extend through
basal growth to lift their blades
above water.  Something like
this may have preceded the first
monocots.

It is therefore not certain whether monocots were initially adapting to aquatic conditions or to harsh seasonal conditions.  They may have been preceded by aquatic dicots that also have the ability to extend their leaves upwards through basal growth of their petioles (leaf stalks).  One theory is that in the ancestral monocots, basally growing petioles eventually lost their conventional leaf blades, and the petiole became wider to become the new kind of blade.


From basic monocots with underground rhizomes, bulbs, and corms, many specialized forms have evolved, including tree-like members of the palm, pandan, and Dracena families. Epiphytic orchids, aroids and bromeliads have rhizomes that creep along tree limbs.  Banana plants use basal growth to lengthen their cylindrical leaf sheaths into a false stem, while Egyptian papyrus lifts a globular crown above water through basal growth of its flowerstalk.  And finally, the giant grasses known as bamboos rapidly reach tree height by regions of basal growth in each of their internodes.

Why are there no moss trees?

The land plants can be grouped into two broad categories:  vascular plants and non-vascular plants (better known as bryophytes).  Most of the plants we encounter in everyday life -trees, shrubs, flowering plants, ferns, etc., are vascular plants, while mosses, liverworts, and hornworts are non-vascular bryophytes, and quite humble in their habits.   

Vascular plants, such as this giant Sequoia,
 can get quite tall because they
possess xylem for upward water transport

 You may have heard the word “vascular” in reference to the vessels of our own circulatory system, and vascular tissues in plants perform a similar function. They consist of parallel sets of pipes: xylem vessels for conducting water upwards, and phloem tubes for moving dissolved foods from one part of the plant to another. Without such tissues, in particular the xylem, water cannot rise very high, and significant upward growth is not possible (see my posting "how does water get to the top of a redwood tree?").  Vascular plants have them, and bryophytes obviously don’t. 

Bryophytes cover every available surface
in the temperate rain forests of Washington
State

Mosses mostly do not exceed more than
a few centimeters in height. The leafy portion
of the plant produces gametes, and a fertilized egg
then develops into the spore-producing plant, which is
just the stalk and sporangium.  From W.H.
Brown, the Plant Kingdom, 1935.

Most bryophytes are prostrate, forming mats on the soil, trunks and branches of trees, rocks, and sometimes tombstones. Those that grow more upright are typically only a few centimeters high, but there are some giants among them that tower above their cousins to dizzying heights of about half a meter!

Liverworts lie flat, except for their reproductive structures.  From A. W.
Haupt,
Plant Morphology, 1953. 

Hornworts are similar to liverworts, but
with different form of reproductive
structures.  From G. M. Smith, Cryptogamic
Botany, 1938.

It seems strange at first that there are no larger mosses.  Nearly every group of organisms has both large and small members.    A common response, even in botany textbooks, is that mosses can't get any taller because they don't have vascular tissues.  More technically it is said, they "failed" to evolve lignin, a resin-like material that strengthens the walls of xylem vessels. 

Xylem vessels serve the same function as drinking straws.  Water is sucked up through them by the force of transpiration.  Imagine a straw made of silk.  It would collapse under the slightest suction and be useless.  The same applies to unreinforced cell walls.

I would suggest that if mosses really needed xylem, it would have evolved.  But we don't even have to pursue that argument because as it turns out, bryophytes couldn't get any taller even if they "wanted to," and it's not for lack of lignin or motivation.


For the real reason, we have to recall that plants have alternation of generations of gamete-producing plants (gametophytes) and spore-producing plants (sporophytes) (see my posting on "the Truth about Sex in Plants").   There are two alternate forms of every sexually-reproducing plant, one that produces spores and one that produces gametes. One is usually large and long-lived, the other small, short-lived, and generally unnoticed. 

In bryophytes, the main plants - the green mats that spread and live for many years - are the gamete-producing generation, just like their algal ancestors.  They cannot  get very tall, because their ultimate task is to release sperm cells and position eggs to receive them. Sperm cells can swim only a short distance but must reach an egg on another plant - a difficult proposition for fragile cells produced on a tree top.  Sperm cells produced on a large  gametophyte tree would be left literally "high and dry."


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Tree ferns are vascular plants, and
 their spore-producing generation is the main plant
that can get quite tall.

The spore-producing plant of a moss, its sporophyte, is a small, ephemeral structure that remains attached to the parent plant - just a slender stalk and a single sporangium.  It gets as tall as it can without toppling over or placing excessive demands on the gamete-producing plant - a few centimeters at most. 

But suppose that tiny spore-producing plant of the moss were to sprout its own roots and start growing on its own.  Then it could get as tall as it wants, because there is an advantage to dispersing spores from greater heights.  Well something like that did happen in the ancestors of the vascular plants, and their spore-producing generation became the dominant conspicuous one, inventing lignin and xylem as a means to become ever taller.  Voila, trees!

The gamete-producing generation of the fern resembles
that of a liverwort, but is even smaller and very short-lived.
From A. W. Haupt, Plant Morphology, 1953.

So in bryophytes, which are indeed well-adapted to creeping around in the shade, the gametophyte is the dominant plant, while the sporophyte is tiny, but in the tall-growing vascular plants, the sporophyte is the dominant plant, while the gametophyte is tiny. 

               

How does water get to the top of a redwood tree?

The upward movement of water more than 100 meters in a redwood or eucalyptus (there is  a traditional dispute between Americans and Australians over who has the tallest trees!) seems to be a gravity-defying task of epic proportions.  Gravity is certainly a factor, and ultimately limits how tall a tree can get, but there are other forces at work that can meet that challenge.  The amazing thing about plants is that the process is largely passive, in the sense that plants expend practically no energy to accomplish it.  There are no muscles and no heart in a tree to pump water upward.  What there is, is basically a gigantic paper towel.

You may recall the ads for the “Bounty Quicker-Picker-Upper”  a decade or two ago.  In the ads, these paper towels quickly absorbed any spilled liquid.  You can take a paper towel (even a cheap slower-picker-upper), roll it into a tube, and stick the end in water. You’ll quickly see the water begin creeping up the paper towel.  We see the same gravity-defying process in plants.  What is a paper towel made of?  Wood!  And the magic ingredient in both a paper towel and a living plant is cellulose. 

Every plant cell is wrapped in a layer of cellulose – the cell wall.   In wood, the cells of the xylem die after laying down strong cellulose walls, leaving the latter as narrow, empty conducting tubes.  These tubes line up so as create masses of interconnected passageways through which water can move freely.  So a tree trunk is essentially a massive, non-living, interwoven paper towel.   

But what is the interaction between cellulose and water?  What is the force that can overcome gravity?  The short answer is magnetism.

Magnets can defy gravity by lifting nails and other iron objects.  No, neither cellulose nor water is made of iron, but the forces involved are similar.  A molecule of water is electrically charged.  Imagine each molecule as a Mickey Mouse head.  The face is an atom of oxygen, and the ears a pair of hydrogen atoms.   The two types of atoms are bound together by sharing electrons, but the oxygen atom holds onto the electrons much more tightly than the hydrogen atoms do.  the result is that the electrons hang out around the oxygen atom most of the time, giving its side of the molecule a net negative charge.  The two hydrogen atoms are left with a net positive charge as they are visited less often by the electrons.   Thus water molecules are thus like tiny magnets and tend to stick together.  This actually is what makes water liquid, rather than a gas, at room temperature, and which accounts for a lot of other properties that we don’t have time to review here.

It so happens that the complex surface of cellulose fibers also have positive and negative charges, and so attracts water molecules.  In a paper towel, water molecules are pulled into every available niche in the cellulose matrix.  Those at the top are pulled into higher niches, which pulls more water molecules from the bottom.  Narrow spaces within the matrix also fill with water molecules, which attract and pull each other in.  This happens until the towel is saturated and there is no room for any more water. 

The water molecule (A) consists of a negatively charged oxygen atom and two positively charged hydrogen atoms.  This causes them to stick together in chains (B) and to the walls of cellulose fibers (C).

If we return to our rolled up paper towel with the end sitting in the water, we can watch something else happening over time.  The paper towel only gets saturated at the base, and even remains fairly dry at the top.  But given enough time, the container of water it is standing in will completely dry up.  Of course what is happening is that the water that has moved up the column continually evaporates when exposed to the air, leaving spaces for more water molecules to move up.  The result is a steady stream of water moving upward, drawn by both evaporation and the electromagnetic attraction of water molecules to the cellulose and to each other. 

This is the process of transpiration, which is powerful enough to lift water and dissolved minerals to the top of a tall tree.   It continues as long as water is evaporating through the leaves.  In extremely humid weather, or if the leaf pores (stomata) shut down for the night, the stream is suspended in place until evaporation resumes again later. 

I should note that a tall tree, if dried out, cannot start this process from scratch.  Gravity will stop the electromagnetic movement of water long before it can reach the evaporation zone of the leaves.  The transpiration stream develops in a germinating tree seedling and is maintained and strengthened as the tree grows, but if that stream should be broken (interrupted by extensive air bubbles) in an extreme drought, it cannot be repaired and the tree will die.   Only small plants like mosses can completely dehydrate and recover when wet conditions return. 

What is a vegetable?

The morning headlines are telling us that Congress has declared pizza to be a vegetable.  Pundits around the world are reacting to this news from every conceivable point of view.  I will leave the nutritional arguments to nutritionists, the political arguments to the Occupy Wall Street movement, and making fun of congress to Jon Stewart.  So what is the botanical perspective?

First, one must note that congress did not actually declare pizza to be a vegetable. That was a revision made by the “media,” to make the story more sensational.  Congress was simply legislating that tomato sauce is a vegetable, and that the amount found in a pizza qualifies the latter as a balanced meal (something college students have always known).  Again it is basically a nutritional/political issue, but perhaps I can address the point of whether or not tomato sauce is a vegetable.

Botanically, there are widely varied definitions of the word “vegetable.” It is not an officially defined technical term.   The plant kingdom has at times been referred to as the “Vegetable Kingdom,” and if you’ve ever started out a round of 20 Questions by asking “animal, vegetable, or mineral,” you’ve employing that definition.  So by that definition, anything that comes from a plant is a vegetable, including wheat flour.  A pizza, if totally vegan, could therefore could be classified as a vegetable, along with donuts, chocolate chip cookies, and fruit loops.  Now if the members of congress had boned up on their botanical history, they could have used that definition and made this whole debate a lot simpler.

It is more useful to break down the different categories of plant parts, and consider their role in nutrition.  In terms of the parts we eat, we can consider the following classification:

1.       Leaves and stems – these are your hard-core vegetables, low in carbs, high in vitamins, minerals, and fiber.  This is what nutritionists are talking about with respect to improving the quality of school lunches.

2.       Seeds – these are the structures containing embryos and stored food for the next generation of plants.  They include peas and corn, which are considered “starchy vegetables,” along with various kinds of beans, which are sometimes considered vegetables, sometimes not.  Actually a kernel of corn (maize to the rest of the world) is botanically identical to wheat, oats, barley, etc., and we don’t consider popcorn, corn muffins, or fritos to be vegetables, so corn is marginally a vegetable at best.  The “baby corn” popular in oriental cooking works better, because it hasn’t stored up the starchy reserves yet. (technically, corn, wheat and other cereal grains are single-seeded fruits with a very thin, hard fruit wall – but that’s getting into botany geek territory and we don’t have to go there).

3.       Starchy roots and other underground structures.  Many plants store food underground for use in the next growing season.  They are loaded with starch, and so can contribute a lot of calories.  Carrots, beets, and radishes are true roots, as are sweet potatoes, but white potatoes are specialized underground stems called tubers.  Onions might have the best claim to be true vegetables, since they consist of compact cluster of specialized leaves, but they also store carbs and are calorie-rich. 

4.       Fruits – technically a fruit is a structure that encloses seeds in plants, but many fruits that are not sweet are used as vegetables: peppers, eggplant, okra, cucumbers, squash, green beans, and … tomatoes! So botanically speaking tomato sauce is a fruit product, but we yield to common usage on this point, because tomatoes are low in sugar, high in vitamins, lycopene, etc.  Tomato sauce can therefore be considered at least a vegetable product, along with ketchup. The problem with these is what else is in them – like up to 60% of your daily allowance of sodium in a cup of tomato sauce.

So, from a botanical perspective, you can use the word vegetable in almost any way you want in reference to the parts of a plant, and so it becomes irrelevant nutritionally unless carefully qualified.   Nutritionists should have the final say, because it’s what’s in those vegetables and vegetable products, and the overall balance between the different food groups that is the issue.

The truth about sex in plants

It may surprise you to know that plants reproduce by sperm and egg, the same as animals. Well not quite the same, of course, since plants can’t move around or physically interact during the process.    This was a huge problem for the first plants to creep up onto dry land.  Their ancestors were green algae, who as a group mostly release their sperm cells (and sometimes their eggs as well) directly into the water to fend for themselves.  In fact many aquatic animals including sea stars and most fish, follow the same strategy.    Releasing sperm and egg cells into dry air, however, just doesn’t work!   

The seed plants eventually solved the problem through the invention of pollen grains -  private limousines  in which  sperm cells ride comfortably through the air. Pollination of pinecones and flowers is what you usually hear presented as the sex life of plants.   But the technology of pollen grains was long in coming.  How did the first plants manage with their sperm cells flopping around on dry land?

Sperm cells were indeed naked and vulnerable as they made their first journeys on land, and they still are in some modern plants like mosses and ferns.   Mosses and ferns are, however, numerous and diverse.  In the sex department, they apparently do just fine, thank you very much.  But how exactly? We’ll focus on ferns for now.  Mosses do things a bit differently, and we’ll return to them in a future post. 

 Like their aquatic ancestors, the sperm cells of ferns must swim through water to find an egg.  This can only take place when the soil is quite wet, and even so, sperm cells can’t travel very far - a few centimeters at best, so suitable mates must be quite close.  However, even if sperm and egg cells were dropped from the fronds of adjacent ferns, any two plants within sperm range of each other would most likely be siblings or at best  cousins, (or at worst  branches of the same plant!) Reproduction could take place only between nearest neighbors, generation after generation.  Plants would exist in pockets of incestuous interbreeding, and everyone knows how bad that would be.  The whole evolutionary purpose of sex is to create genetically diverse populations through breeding among widespread and genetically different individuals.  How did the early land plants escape from this trap?

The solution is something called alternation of generations.  This involves separating the reproductive process into two phases, with a small alternate “individual” as a go-between.  So there is the main plant that we see every day, and a small temporary plant that lives just long enough to complete its reproductive function.   In the fern, the main plant produces not sperm and egg but spores.  These can be seen at certain times of the year, produced in dense clusters of spore chambers (sporangia) on the lower surfaces of the fronds.  The spores are hard and dry and can be dispersed great distances by the wind.  The spores of different individuals, sometimes from widely spaced populations can whirl around in the wind and by chance land together in a suitably moist piece of ground.  

The tiny gametophytes of ferns are
typically little more than 1 cm long
and lie flat against the ground, Sperm
and eggs are produced in tiny
on the undersurface, among with the
root-like rhizoids seen here. from

Haupt, A. W. 1953. Plant Morphology.

McGraw-Hill. New York.

These genetically mixed populations of spores then germinate into the tiny alternate plants that are small enough and close together enough for sperm cells to swim between them.  So it is these alternate plants (the gametophytes) that conduct the sexual phase of reproduction.    In fern gametophytes, sperm cells and eggs are typically produced at slightly different times, so as to avoid the dreaded self-fertilization.  In some species individual gametophytes produce either sperm or eggs, not both – an even better insurance against self-fertilization. 



The large, ordinary plants that we
recognize as ferns are the
sporophytes.  Spores are produced on the
undersides of the fronds.



The big fern plants (the ones we actually see when we walk through the woods) are called sporophytes, because their reproductive task is to produce spores.   Ferns sporophytes have two sets of chromosomes in every cell (they are diploid) like animals, and the special cells on their fronds that produce spores undergo meiosis, a special type of cell division that results in cells with just one set of chromosomes (they are haploid).  So the spores, the gametophytes, and the gametes are all haploid.  When sperm and egg unite, they produce a diploid zygote, which develops into a new diploid sporophyte.   In ferns and seed plants, it is the sporophytes that get large, living often for many years, while the gametophytes are very small and ephemeral.  In mosses and liverworts that situation is reversed, but I’ll get to that in another post.

By separating the reproductive process into two phases, or “generations,” plants overcome their own immobility as well as the limited mobility of their sperm cells, and produce genetically diverse descendants.   Long-distance dispersal and mixing of different genotypes is accomplished by inert spores, and the joining of sperm and egg is then accomplished over a short distance by the tiny gametophytes.   Animals that move about and choose their mates don’t need an intermediary gametophyte.  They produce sperm or egg directly through meiosis, and the males actively deliver the sperm to the females themselves.

But how does all this translate into pollen grains?  Seed plants still employ tiny gametophytes for producing sperm and egg, but these are even tinier and harder to see.  To make a long story short, in the transition from fern-like ancestors to the first seed plants, specialized, sperm-producing gametophytes shrank down into tiny 2- or 3-celled structures that remained within the wall of the spore.   In other words, a pollen grain is a spore that contains a tiny, prepackaged male gametophyte.  The female gametophyte, containing an egg, forms within the embryonic  seed (ovule).  The pollen grain, like spores in general, is dispersed some distance from its parent.  When it lands in the vicinity of an ovule on another plant, a pollen tube develops that carries the sperm cells directly to the egg.   Neither the sperm-producing gametophyte nor the egg-producing gametophyte lives on the ground, and the sperm cells don’t have to swim through wet soil.   This allows seed plants to live and reproduce in a greater variety of habitats than their predecessors.

The essential characteristics of plants

The following is intended as a very concise summary of the characteristics of plants that distinguish them from animals and other organisms.  It is provided as a guide to students, instructors, and the botanically curious who want to grasp the big picture of plant life.  My book, "Plant Life: a Brief History," expands on these themes in an evolutionary context, exploring how and why plants are the way they are.

1.       Plants are photosynthetic.  Plants are primarily oxygenic photoautotrophs, i.e. they conduct photosynthesis in which oxygen is released as a byproduct.  They share this fundamental metabolism with cyanobacteria, various organisms referred to as algae, and even a few animals.  Photosynthesis in plants and algae occurs in chloroplasts, which are descendants of cyanobacteria captured through primary endosymbiosis. 

2.       Plants are multicellular, primarily terrestrial organisms descended from green algae.  The formal Plant Kingdom (clade Embryophyta) is descended from the green algal group Charophyta, and consists of complex, multicellular, mostly terrestrial plants in which early embryonic growth is protected and nurtured in special chambers on the parent plant. 

3.       Plant growth is indeterminate and adapted to gather diffuse resources.  The resources required for photosynthetic life, including light, carbon dioxide, water, minerals, tend to be distributed diffusely, requiring broad, antenna-like systems, above and below ground, to gather those resources.  The diverse forms of plant architecture reflect different strategies for optimizing those resource gathering systems.   Most plants have no fixed size or shape, though some have a well-defined lifespan.  They expand indefinitely, adding new photosynthetic and absorptive organs throughout their life. 

4.       Shoots consist of simple repeated units exhibiting serial homology.   A shoot is a young section of stem with leaves or other derived organs produced serially through growth at its tip.  Leaves are usually determinate structures of fixed size, shape, and lifespan, and are attached at points along the stem called nodes.  Nodes may be separated by sections of stem called internodes.   An axillary bud, from which a branch shoot may arise, is situated in the axil (basal angle) of each leaf.   Leaves may be modified into bracts, tendrils, spines, or floral organs through modification of a common fundamental development plan (i.e. these structures are serially homologous).

5.       New tissues and organs are formed at meristems.   Growth in plants is localized in specialized tissues called meristems.  The meristems for primary growth (apical meristems) are located at the tips of shoots and roots, and in plants with elongate stems, cell division and expansion may continue for some time within the internodes.  Branching in stems is achieved by the growth of axillary buds, and in roots by the formation of new meristems deep within root tissues.   The stems and roots of many plants expand in thickness through cylindrical secondary meristems that produce wood and bark.

6.       Plants are hydrostatic systems.  Plant cells have rigid cell walls made primarily of cellulose, which limit cell expansion caused by osmosis.  This results in turgor pressure, which plays a part in cell growth, tissue expansion, movement of plant parts, maintenance of organ rigidity, and transport of food, water, and minerals.  The cell walls are outside of the living protoplasts, and along with intercellular spaces and hollow conducting cells, collectively create a non-living matrix (apoplast) through which water may move from one part of the plant to another.  Cytoplasmic strands (plasmodesmata) connect the living protoplasts of plant cells, resulting in a second network (symplast) through which fluids including dissolved sugar and other organics, may also move from one part of the plant to another, often through active manipulation of osmotic forces.   A large central vacuole contains water and other stored substances, and helps regulate cytoplasmic water and solute content. 

7.       Plants have complex reproductive cycles involving alternation of generations.  As non-motile organisms, multicellular plants are dependent on external factors, such as wind, water, or animals, to complete sexual reproduction and disperse their genetic progeny to distant locations.  In the simplest land plants, sperm cells swim to eggs through water, but dispersal to new locales is a separate process achieved by airborne spores.  As the long-distance dispersal of spores and the short-distance travel of gametes have very different requirements, plants typically alternate between two distinct multicellular bodies, or generations.  A diploid spore-producing body (sporophyte) produces spores through meiosis, and is typically tall so as to launch the spores for effective dispersal by air currents.  Spores germinate into haploid gamete-producing bodies (gametophytes), which remain small and close to the ground, so that sperm cells can be released into films of surface water and need travel only a short distance to unite with an egg on a genetically different plant.   In seed plants, sperm-producing gametophytes are tiny and remain within specialized spores (pollen grains) that are carried by wind or animals to the embryonic seeds (ovules) where the egg-producing gametophytes are located. 

8.       Plants defend themselves without moving.  To protect themselves against predation by herbivores, plant organs produce layers of fibrous or stony tissues, cover themselves with superficial hairs or scales, arm themselves with piercing spines, thorns or prickles, produce repellent or toxic chemicals, or provide food and shelter for animals that actively defend the plants.   Some plants invest little in defense, and instead rely on rapid replacement growth.