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.  

Why are the seeds on the outside of a strawberry?

What appear to be the seeds on the surface of the strawberry
are actually multiple tiny fruits.

[For more fascinating adaptations of fruits and seeds, see chapter 7 of my book, Plant life - a brief history]

I should begin by putting both “seeds” and “fruit” in quotation marks, because all is not what it seems when it comes to the strawberry.    The short answer to the question is that the seeds are right where they belong – inside the fruits!

The long answer requires a short lesson in flower structure.  At the most basic level, flowers consist of four series of organs: sepals, petals, stamens, and carpels (sometimes also called pistils).   Sepals are typically green and surround the rest of the flower in bud, and the petals are typically the most conspicuous, often colorful and/or fragrant, parts of the flower inside of the sepals.   Stamens bear the pollen, which will be carried away by wind or animals to another flower, where they will release sperm cells.  The eggs awaiting those sperm cells are located inside embryonic seeds called ovules, and the ovules in turn are inside the carpels.  The hollow part of the carpel that contains the ovules is referred to as the ovary (the more accurate term, “ovulary,” never caught on).

The flower of a strawberry. "r" points to the receptacle, which will swell to become the edible part.  The small ovoid structures on the surface of the receptacle are the carpels that contain one embryonic seed each, and will become the hard-walled true fruits as the seeds matures.  Illustration from the classic botany text by Hill, et al.

In some flowers, including the most ancient, flowers include a number of separate carpels.  This true in many members of the Rose Family (Rosaceae), including strawberries (genus Fragaria) and blackberries (genus Rubus).  The carpels occupy the center of the flower and are mounted on a rounded platform called the receptacle.  Each of the carpels in these plants contains just one embryonic seed.    Among flowering plants in general, there are many variations on carpel structure.  In most, carpels are fused together into a compound pistil, and the number of seeds produced within can number from just one to hundreds of thousands. 

The word “fruit” is used quite differently by botanists and consumers of said objects.  What the botanist calls a fruit is technically the tissues of the carpel/pistil/ovary that expand as the seeds within them mature.  What we call fruits in the grocery store and cookbooks are generally plant parts emerging from flowers that are juicy and sweet, but not necessarily part of the carpels.    Sometimes, these two definitions coincide, but sometimes not.  We tend to think of tomatoes, egg plants, bell peppers, okra, and green beans as vegetables, but they are technically all fruits because they contain the seeds.  Kernels of maize, whole oats, and grains of whole wheat also fruits (with thin, hard ovary walls), while the juicy parts of strawberries and apples that we consume are not technically or entirely part of the fruit.

In blackberries, the fruit is an
aggregate of many simple fruits
mounted on a common receptacle,
as in a strawberry, but the individual
carpels become fleshy rather than the
receptacle.

The flowers of strawberries and blackberries are quite similar, but how they develop into fruits is quite different.   In strawberries, the receptacle expands into the tasty succulent tissue that we crave, while the tiny dry fruits, which each contain one seed, sit on the expanding surface.    In the blackberries, the carpels themselves expand into small, juicy, seed-containing globules, and so are true fruits. 

An apricot, like a plum, cherry, or peach,
 develops from a single carpel.

Apples (genus Malus) and plums (genus Prunus) are also in the Rose Family, and their flowers are similar to those of strawberries and blackberries.  Apples are different in that tissues from the receptacle expand on the outside of the carpels to become the edible portion. If you bite too deeply into the apple (or pear) you encounter the hard inner wall of the true fruit and the seeds within.   The plum (or apricot, cherry, or peach) is more like the blackberry, except that there is only one carpel in each flower.  The carpel wall expands to form the edible tissue, and so is a true fruit.

Plants and animals and kleptoplasts – oh my!

What is a plant?

Everybody knows what plants are.  They’re green organisms that set down roots, spread out leaves and turn sunlight into sugar.  So a simple statement like “plants are the living organisms of the Earth that photosynthesize” ought to do for a definition.  That would probably suit ecologists, for it simply and clearly defines a distinct class of organisms according to their role in the ecosystem.  But if you have already glanced uneasily at the length of this post, you know that it’s not going to be that simple. 

                Everyone will agree that the trees, shrubs, and herbs that populate our gardens and forests are plants, but  the inclusion of other plant-like organisms is debatable.  What about algae, for example, or fungi and other plant-like organisms that don’t photosynthesize?  All have been called plants at one time or another.  The word “plant” is a common term, and there are no rules for what it may include, and so it has been used in a variety of ways.  Nevertheless, many of my colleagues argue that the term should be used in a fairly restrictive sense, almost as if it were a formal taxonomic category, e.g. as in the “Plant Kingdom.”  Ecologists with some savvy of the complexities of biological classification might simply describe the first link of the global food chain as consisting of “photosynthetic organisms” and thereby avoid taxonomic confusion. 

Modern biological classifications are based on relationship rather than ecological profession.  A Smith is always a Smith, even if a member of that family decides to grow cabbages rather than shoe horses.  Likewise, some plant families contain rogue members who have abandoned solar sugar-making for other life styles.  My goal here is to clarify the modern definition of “plant,” but along the way have some fun looking at various plant pretenders and why they have been excluded from the Plant Kingdom.



The snow plant, Sarcodes sanguinea,
 growing in the Sierra Nevada, California.
  This is a non-photosynthetic member of
the Blueberry Family, Ericaceae.

                Indian pipes and snow plants, for example, live like fungi, extracting energy from organic matter, yet are closely related to blueberries and Rhododendrons (family Ericaceae).  Fungi have been excluded from the Plant Kingdom because genetically they are more closely related to animals.  Snow plants and Indian pipes, however, have flowers similar to those of the blueberry and DNA sequencing confirms that they belong in the same family.  They are still plants.  Many plants in a variety of families have similarly found it more lucrative to siphon off sugar made by other organisms than to make their own.  Wikipedia lists 57 genera of plants, including both eudicots and monocots, that obtain organic energy from underground fungi.  They are particularly common among orchids.  Others are parasitic directly on the stems or roots of other plants.  Dodder is a parasitic  vine in the morning glory family (Convolvulaceae), and the kiss-mandating mistletoe is a hemiparasite as it does some photosynthesis.  The largest flower in the world, Rafflesia, is also a parasite.  All of these are plants because they are related to “normal” plants.

Is a sea slug a plant?

A photosynthetic sea slug, Elysia chlorotica. 
Photo by Skip Pierce, University of South Florida.

                OK, so parasitic plants are a minor deviation that we can deal with, but there are more serious photosynthetic criminals afoot. There are organisms that have no business being photosynthetic at all: sea slugs for example.  Sea slugs are mollusks - relatives of snails, clams, and squids.   They are clearly animals.  They move around and eat stuff.  They have nerves, muscles, eyes, etc.  Nothing could be more unrelated to plants than a sea slug.  Yet there are certain species of sea slugs that are “photosynthetic.”  The most extensively studied are in the genus Elysia. Their tissues are filled with chloroplasts that make sugar and release oxygen.  The chloroplasts can sustain the animals for prolonged periods of time when there is nothing to eat.  Doesn’t that make sea slugs plants?



If you’ve read about such creatures before, you’re thinking to yourself “but they cheat!  They don’t have chloroplasts of their own, they steal them from the algae they eat.”  And you’d be correct.  Sea slugs are among a surprisingly long list of “kleptoplastic” organisms, a term coined by biologist Kerry Clark¹.   These animals eat algae but retain their functioning chloroplasts (kleptoplasts) within certain of their cells, and even have evolved a flat, leaf-like shape.²  The chloroplasts  eventually wear out and are replaced through ingestion.

Other animals, such as nudibranchs, giant clams (Tridacna), corals, a sea anemone (Aiptasia) and even a spotted salamander, Ambystoma maculatum - the only known photosynthetic vertebrate³, maintain whole algal cells (called zooxanthellae), within their tissues, and are able to share their photosynthetic product.  That is a slightly different sort of symbiosis, but the net result is similar.  In all these animals, the relationship is temporary.  The young animals are not born photosynthetic, and must they ingest  algae in their food to obtain their first chloroplasts.   Zooxanthellae may persist indefinitely as self-reproducing populations within their hosts, but can also be expelled.  Photosynthetic animals are photosynthetic in the same sense that a greenhouse is photosynthetic.  Photosynthesis is being done by what’s inside, not the structure that houses them.

In “true plants,” chloroplasts reproduce under the control of the host cells and are passed on from generation to generation as the cells divide.  Chloroplasts have been integral parts of plant cells for hundreds of millions of years, and could not live on their own.   Non-photosynthetic plants still have ‘plasts’ (technically plastids), but they have been modified for other purposes, such as storing pigment (chromoplasts) or storing starch (amyloplasts).

Plants therefore cannot be defined simply by the presence of photosynthesis.  Animals are not plants, even if they are photosynthetic, and non-photosynthetic organisms are plants if they are related to plants.   What then are the true plants?  Certainly the mosses, ferns, gymnosperms and flowering plants are, but who else?

What about algae?  They have permanent, reproducing chloroplasts that are passed on from generation to generation.  Are they plants?  They certainly have been considered part of the Plant Kingdom in the past, but modern studies of evolutionary relationship, involving structure, chemistry, and DNA sequencing, have revealed a very complicated picture, with some groups of algae closely related to land plants and others not related at all.  Keep in mind that by current taxonomic standards, a recognized group of organisms must be related by common ancestry.

The term “algae” is generally applied to photosynthetic members of the broad grab-bag of organisms referred to as protists.  Protists are generally simple and generally aquatic, and do not protect or nourish their embryos.  Some organisms that fit that description have been revealed to be kleptoplastic.    There are a number of predatory, animal-like protists (“protozoans”), including some foraminifera, dinoflagellates, radiolarians, and ciliates, who incorporate small algae or chloroplasts into their cells, just like sea slugs.  They generally have non-photosynthetic relatives, and so can be excluded on the same grounds as sea slugs.

Now for a more troublesome case: certain species of the unicellular protist genus Euglena have permanent, reproducing  chloroplasts within them, and so might be considered plants.  Some related euglenoids, however, are non-photosynthetic foragers, consuming food like miniature animals.  Whether these organisms are plants or animals has puzzled biologists for centuries.  Both zoologists and botanists have classified them within their respective kingdoms.  Are the photosynthetic euglenoids kleptoplastic predators or plants?  It turns out that their ancestors acquired chloroplasts the same way that sea slugs did, but the fact that their chloroplasts are permanent and passed on from one generation to the next makes these organisms decidedly more plant-like.  Their kleptoplastic origin from unrelated predatory cells, however, banishes them from the Kingdom. 

What about other algae?  Brown algae, like the giant kelps that form “forests” off the coast of California are surely plants, right?  Sorry, no.  They are the greatest plant pretenders of all.  Brown algae, and their ubiquitous cousins the diatoms, have fully integrated, reproducing, heritable chloroplasts, like the photosynthetic euglenoids, but these too were stolen, probably from red algae, by their early ancestors.   Dinoflagellates, the group that includes the organisms responsible for “red tide,” are even more complex, having multiple kleptoplastic events in their history.  The ancestors of all these kinds of algae were predators that obtained kleptoplasts, and again not related to the trees and grasses of the terrestrial environment.  Modern classifications are based on the relationships of the host cells, not on the chloroplasts they contain, so all algae that originated through kleptoplasty are excluded from the plant kingdom.

So what’s left?  We must see where chloroplasts came from in the first place to finalize the roster of true plants.

The first chloroplasts

Endosymbiosis is the creation of a close, mutually beneficial relationship between two previously independent organisms, with one residing inside the other.   Delving deep into evolutionary history, we find that the first chloroplasts originated from whole photosynthetic bacterial cells related to modern Cyanobacteria.  This is referred to as primary endosymbiosis, to distinguish it from kleptoplastic secondary endosymbiosis.  Mitochondria, incidentally, also resulted from a primary endosymbiosis between aerobic bacteria and ancient eukaryotic cells. 

The ancient photosynthetic cyanobacteria that became the first chloroplasts left many free-living relatives that are still abundant today.  Their common ancestor invented oxygen-releasing photosynthesis over 3 billion years ago, and the chloroplasts in all photosynthetic organisms today are descended from them.  Cyanobacteria (previously known as “blue-green algae”) were once classified in the plant kingdom, along with all groups of algae, fungi, and well just about anything that clearly wasn’t an animal.  But since they are bacteria with simple prokaryotic cell structure, they are even further from being related to plants than sea slugs, euglenoids or brown algae.  (Prokaryotes have simple cell structure without the nuclei or other complex internal organelles found in the eukaryotic cells of plants, animals, fungi and protists)  Bacteria and eukaryotes are classified in different domains (bigger than kingdoms).   Once again, organisms are classified according to their main cell structure, not what might be in them or how they make a living.

So, who is left (we are almost there!)?  Evidence suggests that red algae, green algae, and terrestrial plants form a natural group descended from the first plant-like cells with captured cyanobacteria.  Is this our Plant Kingdom?  Together, they are officially named Archaeplastida ("ancient plastids"), without committing to a particular rank.   Logically and according to the rules, this could be considered the Plant Kingdom. Or we could restrict membership to just part of that large clade: the green algae and land plants, together known as the Viridiplantae ("green plants").  Most practicing biologists today, however, prefer to include only the complex, primarily terrestrial plants, the Embryophyta ("embryo plants"), in the formal Plant Kingdom, and that’s where it sits at the present.   The alternate suggestions are quite valid, however. Even with classifications based on true relationship, names and ranks are arbitrarily assigned and there are different ways that can be done.  I will most likely devote a future post two to the ins and outs of formal classification.

We could turn this completely around, however.  The chloroplasts of all modern plants, algae, and photosynthetic animals are direct descendants of cyanobacteria.  No eukaryotic organism evolved photosynthesis on its own, not even those officially designated as plants.    An oak tree is therefore no more legitimately photosynthetic than a sea slug!  One could argue then that cyanobacteria and their descendant chloroplasts are the only true plants, the cells that contain them are just packaging.

  Well, one could argue it, but in modern classification, the packaging takes precedence.  Eukaryotic organisms are classified according to the structure and nuclear DNA of their cells, not according to the chloroplasts they contain.  Though their descendants are found in every photosynthetic eukaryotic organism, the cyanobacteria cannot be classified with any of them.  

1.       Clark, K. B., K. R. Jensen, and H. M. Strits (1990). "Survey of functional kleptoplasty among West Atlantic Ascoglossa (=Sacoglossa) (Mollusca: Opistobranchia).". The Veliger 33: 339–345.

2.       http://www.seaslugforum.net/showall/elyschlo (see this site for more pictures and discussion of photosynthetic sea slugs)

3.       http://www.mnn.com/earth-matters/animals/stories/salamander-is-worlds-first-photosynthetic-vertebrate

Welcome

This is a new blog site, and I am still learning the ropes.  So bear with me as I tweak the format and add some links.

From the title, you might guess that this blog site will be devoted to Botany, a sort of Botany classroom and discussion session.  Botany is the study of  plants - their structure, function, evolution, and classification.  I will post short blogs on specific topics, as well as "field trips" with pictures of plants growing wild in various places I have been over the years. As blogging is as much about opinion or interpretation as it is about facts, this will also be a place also to discuss theory, issues, and controversies in the field of botany.

 I should note that Botany is not the same as Horticulture or gardening.  A horticulturist will tell you how to grow strawberries, a botanist will tell you why the seeds of a strawberry are on the outside of the fruit instead of inside where they belong.  There are thousands of places on the web where you can get horticultural information, so I will not provide any here.  Knowing the basics of botany, however, will lead to understanding of horticultural practices, and if you have a horticultural issue that can be addressed by looking at some botanical principle, then I will respond in that way.

I have taught Botany for many years, beginning as a post-doc at Cornell University, and for many years at the University of South Florida in Tampa.  I hope this site will be useful for both students and fellow instructors, and I hope it will be entertaining.

Finally, in choosing the title for this blog, I did not intend to imply that I am the only botany professor on the web, or that you should listen to me more than any of my hard-working and often underappreciated colleagues working at universities, colleges, botanical gardens, and herbaria around the world.  The domain name was available, and I adopted it as a description of what I do, but by all means explore the vast world of botanical websites and blogs that is available to you.  There is much to learn and much to enjoy.  I will pass on links of some of my favorite sites, once I figure out how to insert them.

I will be back soon!