Why are there so many kinds of plants?

(first published at biology-online.org, updated with additional photos here)

Estimates vary, but there are about 300,000 named species of plants, with more being discovered daily.  There may ultimately be as many as 500,000, if and when all are catalogued.  Some botanists include some 10,000 species of red and green algae in such estimates, but others include only the land plants.  Either way, it’s a lot.   Theoretically, each species differs from every other enough to create a unique niche for itself.  Species with very similar niches in the same location will compete with each other for resources, and the stronger species will drive the other to adapt to a different niche or else go extinct.   Hence, “no two species can occupy the same niche.”   Yet it is sometimes difficult to tell what is unique about each species, and why each maintains a seat at life’s table. 

I’d like to do a thought experiment to get a picture of how many possible plant niches there might be.  Imagine a multidimensional “niche space,” each dimension representing a variable plant characteristic.  Suppose first that there were just one dimension, such as a gradient of temperature regimes running from the equator to the pole.  For simplicity, let’s say that this gradient is occupied by 10 different species, each specialized to make the best use of a particular combination of summer warmth and winter cold.  In reality of course there would probably be more, but we’ll keep all of our hypothetical niche dimensions at 10 for easy calculation.

Now let’s add another dimension, let’s say a moisture gradient.  At the equator, we might have 10 different plants, adapted to habitats ranging from evergreen rain forest to barren desert.  There would likewise be 10 such possible niches for each of the other temperature regimes, and so in this 2-dimensionsal niche space there would be a possibility of 100 different plant species worldwide.  Remember that in this simple array the plants differ only in their temperature and moisture requirements.

Now we can add a 3^rd  dimension for growth form: trees, vines, shrubs, epiphytes, etc.   In biological communities one growth form creates subhabitats for other growth forms.  Trees create shade for understory plants, as well as support for light-seeking vines and epiphytes.  So, multiply our previous niche matrix by 10 again, and you have room for 1,000 species of plants in the world. 

Adaptations for pollination and seed dispersal create still more niche dimensions.  For flowers adapted for specific pollination vectors (e.g. by bird, butterfly, bumblebee, wind, etc.) and for seeds adapted for different means of dispersal (bird, wind, mammal fur, rodents, etc.) add two more dimensions, bringing us to 100,000 different possible kinds of plants. 

There are many other ways in which plants differ from one another: how they protect themselves from herbivores (spines, fuzzy coverings, toxic secretions, etc.), leaf shape and texture (for balancing light reception,  CO₂ absorption, keeping cool, and avoiding water loss).  Each of these could be another niche dimension, but if we throw in just one, we multiply our possibilities by 10 again to have over a million.

We’re not quite done yet, however.  We also need to take into consideration geographical isolation, for similar species occupying similar niches tend to occur on different continents and major islands. Assuming just 10 more-or-less isolated geographical regions, each with warm wet lowlands, dry deserts, and cold mountain tops, we’re up to a possible 10,000,000 species in our matrix. In reality, individual mountain tops and isolated valleys frequently contain unique endemic species, adding even more diversity. 

 Even if this estimate is only moderately accurate, we can see that the number of potential plant niches vastly exceeds what actually exist.  Using 10 spots within each niche dimension is of course a simplification; for some dimensions there will be more, and for some there may be fewer.  Subarctic environments, for example, don’t support trees, vines, etc., so the number of growth forms is much fewer. Our model may seriously underestimate the options for pollination.  In a tropical rainforest, for example, there may be hundreds of possible pollinators. And there are niche dimensions that I just brushed to the side.  Nevertheless, the exercise provides a good perspective on the vast range of possibilities.  We in fact start to wonder why there are so few species of plants! 

Most likely, not every possible kind of plant can exist because the total resource “pie” of the ecosystem can only be sliced into so many pieces.  There is a minimum amount of energy, mineral resources, etc., required to maintain each species as a viable, genetically diverse population.  Those that are more aggressive, or that happen to be at the right place at the right time, are the ones that have actually succeeded.

The flowers of Bahia grass, Paspalum notatum, are specialized for pollination by the wind.  The feathery,
Christmas tree-like stigmas stand upright, while the pendant stamens dangle below.

For flowering plants, it is therefore not difficult to see how different sets of adaptations allow so many different kinds to coexist on our planet.  For example, consider three different kinds of plants from the monocot clade.  Their common ancestor was a creeping perennial herb with long, strap-like leaves with parallel veins, and embryos with a single cotyledon.  It had flower parts (sepals, petals, stamens and carpels) in sets of three.   Epiphytic orchids, savanna grasses, and succulent aloes are descendants of the ancestral monocot with quite different sets of adaptations. 

The butterfly orchid, Epidendrum radicans is adapted for
pollination by butterflies with well-developed color vision. Each orchid
species has uniquely shaped and colored flowers that attract a specific
bee, butterfly, hummingbird or other specialized pollinator.

They differ not only in where they grow (tree limbs or rocks for orchids, seasonally dry savannas for grasses, and deserts for aloes), but also most conspicuously in their mode of pollination.  The Epidendrum orchid pictured is pollinated by butterflies, and its tiny seeds are dispersed by the wind.  The grass is pollinated by the wind, and its seeds dispersed by grain-hoarding mammals and birds (and sometimes ants).   The Aloe dichotoma  (a relative of the familiar Aloe vera) is not only a succulent, but has also evolved a tree-like form. Its flowers are pollinated by birds and its flat seeds dispersed by the wind. 

The giant Aloe dichotoma has evolved a unique way to continue thickening its stem, allowing it to become tree-like.

Aloes in general have red, orange or
yellow flowers adapted for
bird-pollination.  The flowers of
Aloe dichotoma are yellow.

The other 60,000 species of monocots exhibit various combinations of these and many other adaptations. Agaves and Yuccas greatly resemble the African Aloes, but evolved in the Americas with different pollinators.  Palms and screwpines (Pandanus) are tree-like but with very different leaf structure and flowers from Aloe, Yucca or Agave, and prefer moister habitats.   Members of the ginger family have flowers that are often as elaborate as orchid flowers, but with a different growth habit and larger, animal-dispersed seeds. The orchid family itself has more than 20,000 species, each with a unique pollinator.  Sedges, cat-tails, and rushes are wind-pollinated like grasses, but adapted to different habitats and modes of seed dispersal.

Yuccas and their relatives live in similar climates
and have similar growth forms as Aloes,
 but  have colorless flowers adapted for pollination by moths.

We don’t have time to consider the other major groups of plants: eudicots, gymnosperms, ferns, mosses, etc., but you get the point.  There are seemingly endless ways for plants to vary – a multidimensional hyperspace of possibilities.  The half million or so living today are the fraction of those possibilities that through aggressiveness or chance maintain a tenuous foothold on our planet.

Mosses of Central Florida 4. Isopterygium tenerum

The asymmetrical capsules of Isopterygium rise on
long stalks from along the creeping stems, and curve to the side.

[For other mosses in this series, see the Table of Contents]

Isopterygium tenerum (Sw.) Mitt. is one of the most common mosses in Central Florida, yet easily confused with others of similar growth habit.  That habit is what we might call "creeping." as apposed to the upright habit of the three previous mosses in this series. Stems lie against the ground or flop over each other in a thick mat.  The numerous delicate leaves spread out, largely on the two sides of the stem so as to face upward and gather the maximum amount of light.  The leaves remain in that spread out condition when they dry.  They  lack a midrib and the cells are uniformly narrow and worm-like, with thick walls.  The sporophytes emerge from along the stem, lifting the sporangia, or capsules, high above the mat of vegetation.  The sporangia are asymmetrical and curve to the side.  When dry, the capsules are constricted below the mouth, which is lined with two rows of teeth.

The opening of the capsule is
surrounded by two rows of teeth, the
outer short, curved and stiff, and the
inner longer, thinner and straight.
These teeth change shape with changes
in humidity, and help to loosen and
eject the spores. This double row of
conspicuous teeth is characteristic
of the Hypnaceae and related
families.

Isopterygium is found throughout Florida, commonly  at the bases of trees: on the lower trunk, roots, and surrounding wet soil.

The leaf cells of Isopterygium and its relatives are narrow and worm-like, with thick, clear walls.
The leaves are simple in structure, lacking a midrib (costa) or specialized cells at the base.

The delicate, feathery foliage of Isopterygium spreads out into a tangled mat.
The flat leaves  maintain their shape and orientation as they dry.

Similar common species include Sematophyllum adnatum and Entodon seductrix.  The Sematophyllum differs most conspicuously when it is dry, as the stems and the leaves curve toward the side. In Entodon, however, the leaves press against the stem when dry, making the shoots resemble little twigs of juniper. Their sporangia are also symmetrical and upright.  The less common Taxithelium planum is similar, but the leaves are covered with many small bumps, or papillae. Schwetskiopsis fabrona, also less common, is
similar, but has papillae at the upper end of each leaf cell, and the leaves are pressed to the stem when dry, like Entodon.

To self or not to self - the story of Drosera capillaris

What does it mean for a plant to "self?".  The expression is short for self-pollination. It's when the pollen of a flower lands on its own stigma or the stigma of a flower on the same plant, resulting in fertile seeds.  It's a feat only plants can pull off as they typically bear both male and female organs on the same flower. However it is not generally recommended even for them.  Most plants take measures to avoid self-pollination.  In some, the stigma is not receptive when pollen from the same flower is released. In others, the stigma recognizes its own pollen and rejects it. There are dozens of different ways to prevent selfing. Why? Because in general, populations that get into that habit become genetically, weak and eventually die out - they are evolutionary dead ends.

There are some times when it pays off , however. It is a way, similar to vegetative propagation, of quickly increasing the numbers of a population, and is especially useful in very transient habitats.

The carnivorous Drosera capillaris is seldom more
than an inch in diameter, and lives only in moist,
acidic, sandy soil.

Enter Drosera capillaris, a tiny carnivorous sundew common in Florida. Their sticky leaves trap tiny insects, supplementing their supply of nitrogen and other minerals in wet and nutrient-poor soils. Almost 200 species of this genus exist, mostly in Australia, southern Africa, and South America, but we have 5 species native to Florida.

These tiny plants appear by the millions in damp soil around swamps and freshwater marshes, seemingly out of nowhere.   Evidently the tiny seeds wait out dry periods in the sandy soil, sprouting when moisture returns.  I have even seen progressions of seedlings appear as the shoreline of a small pond receded over several months of a dry spell.

A few years ago, I became curious about how Drosera capillaris reproduced.  Lifespan can be pretty short for the moisture-loving plants as wet meadows come and go and shorelines recede.  They plants pretty much die when the soil dries out.  They can, however bloom within a couple of months of germination, while only about half an inch in diameter. In more stable moist areas, they can live for several years and get to be around 1.5 inches in diameter.

The flowers are small and white or pinkish, most commonly appearing in early spring.  A successfully pollinated flower can produce dozens of tiny seeds.  Because of the thousands of seedlings that can show up when the soil moistens, I assumed that pollination, presumably by some small bees or flies, was pretty common.

The flowers of Drosera capillaris, lifted well above
the sticky traps, stay open only for
a couple of hours, and only if the sun is shining.

I convinced a few of my botany students to spend a few hours in a large population of sundews during their blooming period in the USF Ecology Area.  Fortunately, it is relatively short.  Flowers typically open up in mid-morning, but only if the sun is shining, and close up by noon.  After several days of observation, one student caught a single small bee that might have been pollinating a flower.  Other than that - nothing.  We saw very few insects come near the flowers, and that has been my experience whenever finding these plants in the field.  So where do all the seeds come from to constantly regenerate the populations of sundews?

An accidental experiment suggested the answer.  I had started some sundews from seed in a pot, and eventually I had a single flower on one of them.  This was miles from any other sundews.  The flower closed up around noon and I figured that was the end of it.  A few weeks later, however, I noticed that a seed pod had formed, and eventually I had a crop of seeds from this single isolated flower.

That seemed to confirm my growing suspicion that these little plants self-pollinated.  Looking again at the flowers, it was clear that as the flowers closed, the stigmas and stamens would be pressed together, resulting in pollination.  It has not been verified by rigorous experiment, but it seems that this is the secret to the great fertility of these small plants and their success at inhabiting a fleeting habitat. Every so often, perhaps, an actual cross-pollination might occur, mixing different genotypes and boosting the genetic diversity of the population.

Mosses of Central Florida 3. Funaria flavicans

Sporophytes arise from the tips of the short upright shoots after fertilization
of the eggs, and are elevated on long stalks.  

[For other mosses in this series, see the Table of Contents]

Funaria flavicans Michx. is easily recognized by its upright growth, broadly ovate leaves with a strong midrib (or costa), and its plump, nodding, slightly asymmetric capsules. It is widespread in eastern North America, and occurs in central Florida on wet sand.  Related species in Florida include F. hygrometrica, which has more strongly asymmetric and narrower capsules, and F. serrata, which has toothed leaves and lacks an annulus around the capsule mouth.

The sporangia, or capsules, are asymmetrical and nodding
to the side. The orangish ring around the capsule mouth is
an annulus.  At the upper left, a younger capsule is still
covered by the cap, or calyptra, which in this genus
 has a prolonged tip.  The ring of short outer teeth (peristome)
around the capsule mouth can be seen at the lower right.

The leaf of  Funaria is one cell thick, and has a thick central costa.
The cells are long-rectangular, with thin walls and many conspicuous chloroplasts.

What's so primitive about Acorus?

So first of all what is Acorus, and why would anyone think it primitive?

Acorus is a marginal aquatic with sword-shaped leaves
typical of monocots, and flowers densely packed onto
a thick spike resembling the spadix of an aroid.  Photo
by H. Zell, posted in Wikimedia commons.

Acorus, commonly known as Sweet Flag, is a widespread marginal aquatic plant native to North America and Asia.  A marginal aquatic is essentially one in which the roots are in soil that is waterlogged most of the time. So they occupy shallow water at the edges of streams, lakes and swamps, as well as marshes and intermittently wet prairies. The leaves of Acorus are flattened into a fan-like arrangement at right angles to the rhizome, similar to many members of the Iris family.  It's flowers, however, are small and crowded onto a dense spike resembling the spadix of an aroid (e.g. Calla lily or Anthurium).  It was in fact classified as an aroid until recently, generally as its most archaic member, but now has been placed in its own family and order.

In this simplified phylogenetic tree of the monocots,
Acorus is the first clade to branch off.  It is the
 sister clade to the common ancestor of all the
 remaining monocots.  Alismatales includes
mostly aquatic plants, but also the mostly
terrestrial aroids (Araceae).  The Commelinoids
 include palms, gingers, and grasses (among others),
while the Lilioides include lilies, irises and orchids.

This new placement is based on recent phylogenetic studies which suggest that Acorus is the sole representative of the most ancient lineage of monocots.  In technical parlance, we say that the single genus Acorus, in its own family and order, is sister group to all the rest of the monocots.   In other words, the ancestor of Acorus split from from the common ancestor of all other monocots 120 million years ago or more.  So we look at it and we say, "so this is what the first monocots looked like!"

Well, probably not.  The truth is that we know very little about what the first members of the Acorus clade looked like, or what kind of habitat they lived in, let alone earlier monocots.  We have no fossils of any of them. The modern genus Acorus is the end points of over 100 million years of evolution, and the lineage certainly has changed in some way over that time, possibly with earlier genera now extinct.

As an analogy, Homo sapiens is the sole surviving species of the family Hominidae, but does that mean that the first members of the family looked or behaved just like us?  We happen to have a pretty good fossil record of our own history, and know that we were preceded by Australopithecus and even more ancient forms.  These early hominids walked upright, but other than that looked a lot more like chimpanzees that like us, and we are separated from them by a mere 5-6 million years.

What we can say is that the common ancestor of the Acorus clade, as it split from the main monocot line, already had the basic features of all monocots:  leaves with parallel veins that grow from the base, a single seedling leaf ("cotyledon") in the embryo, a clonal herbaceous growth form lacking in secondary growth ("wood"), and an entirely adventitious root system (no taproot or other permanent root system).  (See How the Grass Leaf Got its Stripes, January 26, 2012).  Monocots mostly also have flower parts in 3s - 3 sepals, 3 petals, 6 stamens, and 3 carpels. This also appears to have been established by the time the Acorales split off the family tree.

We know even less about the "stem monocots," early monocots that preceded the origin of the Acorus clade and bridged the transition from dicotyledonous plants.   Phylogenetic studies indicate that the monocots originated in or near the Magnolid clade. This clade includes diverse kinds of plants, from the lofty Magnolia trees to the herbaceous or semi-herbaceous members of the Aristolochiaceae and Piperaceae, but no aquatics similar to Acorus.  So there was a kind of "evolutionary wormhole" into which went plants with broad, dicotyledonous leaves and embryos with two cotyledons, and out of which emerged true monocots with sword-shaped leaves and a single cotyledon.

So in what ways might the modern Acorus species be like stem monocots?  And in what ways are they likely more specialized?  Were the first Acoroids  marginal aquatics, like the modern species?  They likely were, since the next clade to branch off the monocot  tree, the Alismatales, consists largely of  aquatics as well. This vast assemblage ranges from marginal wetland plants like Sagittaria to fully-submerged sea grasses.  It includes also the aroids, which range from marginal aquatics to rain forest epiphytes.

The sword-shaped monocot leaf, which pushes upward from a basal growth zone (basal intercalary meristem) appears, however, to have been shaped by a strongly seasonal environment subject to grazing and fires.  It is a leaf adapted for rapid regeneration from an underground stem system.  If the leaf tips of young grass leaves are burnt or bitten off, for example, the basal growth zone can reactivate and restore the blade.

The  unifacial leaves of Iris, Acorus, and many
other monocots are "folded" with the two
halves  in the  lower part pressed together to
forming a flat, narrow sheath.  The upper part
is solid, as if the two sides were glued
together.  Each new leaf emerges through
the sheaths of adjacent leaves.

Also, as monocot seeds germinate, the single cotyledon usually remains within the seed.  It lengthens at its base like monocot leaves in genera, but this serves to push the rest of the embryo deeper into the soil, rather than to push itself into the light.  This suggested to the great evolutionary botanist G.L. Stebbins (1974) that the first monocots were adapted to environments that were alternately very wet and very dry, like a modern savanna.  Seeds germinated when the soil was wet, with the embryo being "planted" deeply in the mud.  The embryo was thus protected from desiccation when the soil later dried out.  So most likely the early monocots evolved within a range of savanna to marshy habitats, where a great many monocots, from grasses to cat-tails, live today.

In what other ways might Acorus either resemble or differ from early monocots?  Most likely the flowers were larger, fewer in number, and more loosely arranged than they are in the present dense spike.   The trend from loose clusters of flowers to dense spikes, as far as we can see, is a one-way trip.  The spike is an efficient reproductive structure, a "super-flower" if you like, that becomes more and more specialized.  It often becomes surrounded by other structures, like the spathe of the aroids, that not only protect the flowers, but also serve to attract pollinators or manipulate their movement over the stamens and stigmas.  It is hard to imagine any selective pressure that would cause that trend to reverse.  Families that contain small flowers in dense spikes include the Araceae, some Arecaceae (palms), Cyclanthaceace, Pandanaceae, Piperaceae, Saururaceae, and to a smaller degree many other families.  In none of these has the trend ever reversed.

Also, we might expect that the carpels (the chambers in which the seeds develop) of the ancestral monocots were separate from one another (apocarpous).  Most ancient angiosperms, including most Magnolids, and most Alismatales (other than aroids), have separate carpels. The three carpels of Acorus, however, are solidly fused together and share a common stigma, the way it is in more advanced angiosperms in general.  Once carpels are fused together, they rarely revert back to separate entities, except sometimes to produce specialized fruit types like the schizocarps of the Apiaceae (carrot family) or the follicles of Asclepiads (milkweeds).  So in this way modern Acorus is more advanced than the Magnolids that preceded it and the Alismatales that followed it.

Tofieldia, in the Alismatales, grows in moist meadows. Its
carpels are only loosely joined together, retain separate
stigmas, and separate as they mature into seed capsules
 (follicles).

What about the fan-shaped clusters of leaves?  The leaves of Acorus are unifacial, which means that the leaf blades appear as if they were folded and fused together, so that both sides are really the back side (abaxial) of a normal leaf.  The apparent fusion of the two sides of the leaf occurs just above the opening of the narrow sheath through which newer leaves emerge.  Leaves like this are common in the iris family, in Butomus and Tofieldia in the Alismatales, and scattered among other monocot families.  Butomus and Tofieldia incidentally both have carpels more separate from one another than in Acorus, and so in that sense are more archaic.

The flowers of Butomus, in the Alismatales, are larger, in
looser clusters, and have separate carpels, and so
are probably more like the ancient monocot flowers than
are those of Acorus.

It is possible that the common ancestor of Acorus and other living monocots had unifacial leaves.  More ordinary open leaves, as found in grasses, cat-tails, daylilies, etc., could have evolved from them by increasing the growth of the open sheath region and reducing the growth of the unifacial blade.  Or the reverse could have occurred, as it evidently has happened multiple times in different families, perhaps as an adaptation to the marginal aquatic environment.  The stiff fan-shaped cluster of leaves may be more sturdy in the face of moving water or flooding.

 Monocot leaf development is highly flexible, as growth emphasis can easily be shifted from one part of the leaf to another, resulting in the vast array of monocot leaves from grass blades to palm fronds, paddle-shaped banana leaves, and heart-shaped yam (Dioscorea) leaves.  This perhaps more than anything, is the key to the great diversity of monocots today.

A very young monocot leaf resembles the hood
of a sweatshirt.   The tissues surrounding the opening
represent the young leaf sheath, with the next younger leaf and
apical meristem (AM) visible within.
The tissues around the opening will form the sheath of
the mature leaf.  A zone of dividing growing cells
(meristem) that forms around the base of zone A will
 form a  cylindrical leaf sheath.  If a meristem  forms
 in zone B, but not zone C, an open leaf sheath flattening
into a long, grass-like blade will develop.  But if growth
 is largely focused at the base of the solid tip (C),
 a long unifacial blade will form.  

A mixture of primitive and specialized features is common is to be expected in surviving members of ancient lineages, the specialized features no doubt the reason why these plants are still around!  The dense flower spike and fused carpels of Acorus, and possibly the unifacial leaf blade, are likely specialized features that have allowed it to not only survive, but still flourish today.



Reference:  Stebbins, G. L.  1974.  Flowering Plants, Evolution above the Species Level.  Belknap Press of Harvard University.  Cambridge, MA.

The long and short of it - the story of internodes

The structural units of a higher plant consist of a node,
an internode, and one ore more buds and leaves at the
node.  These units can be repeated indefinitely to form
a shoot, or modified in various ways to alter the
architecture of the plant.

Plant stems are organized into repeated units consisting of nodes and internodes.  Nodes seem to be where all the action is.  Leaves are attached at the nodes, and most often there is also a bud at the base of each leaf that has the potential to develop into a new shoot.  The new shoot also will develop as a series of nodes and internodes.


Internodes are essentially the sections of stem between nodes. Ho humm - end of story.  Not quite!  The internode in fact is a dynamic part the plant, the part responsible for most actual growth in height.  You can see the lengthening of internodes most dramatically in vines, which are adapted to extend themselves as rapidly upward as possible to reach the light zone.  In vines, cell division and expansion continue in the internodes well below the tip, propelling several sets of expanding leaves ahead of them.  Once reaching the light zone, internodes will end growth sooner, resulting in a bushier crown suitable for gathering light.  Tree saplings also show dramatic internode elongation in the shade, and many more humble plants will send their flowers rapidly upward by means of strikingly elongate flower stalks.

Internodes are quite distinct in most trees and shrubs (left), but are extraordinarily
elongate in vines like the Clematis (right).

The bamboo's elongate internodes are the secrets to its
height.

Bamboo shoots contain all the
nodes of a full stem packed close
together.  Expansion of all the
internodes, more-or-less
simultaneously, results in very
rapid growth.

Bamboos are grasses evolutionarily adapted to compete with trees, and they do so with the most extraordinary internodal elongation of all.   Each bamboo shoot begins as a compact bud, the "bamboo shoot" of culinary commerce.  Within that massive bud, the nodes and internodes for the entire shoot (culm) are preformed, but the internodes are extremely short.  When growing conditions are right, the entire bamboo shoot elongates through simultaneous elongation of all of its internodes, reaching full height in a matter of days.  The legendary "Chinese bamboo torture" consisted of strapping a prisoner over a bamboo shoot just beginning its expansion phase.  The sharp point of the shoot tip pressing against the back quickly became horribly painful.

This Pachypodium, in the Apocynaceae,
has virtually no internodes, but grows
slowly through the accumulation of
tissues at the base of each leaf.

This Neoregelia in the Bromeliad, forms a concise rosette
of leaves.

Many plants effectively have no internodes.  They are very short, or slowly build up trunks just from the bulk of the tissues around the leaf bases.  Palms and cycads are prime examples, as are many desert succulents.  Plants that hug the surface of the ground have short stems with leaves crowded into a rounded pattern referred to as a rosette (named for its resemblance to the crowded petals of a rose).  Strawberry plants, bromeliads, venus fly traps, sundews, and African violets are examples.

This tiny Drosera  rosette is dwarfed by its much larger flowers.  The flower
stalks consist of greatly elongate internodes, in contrast to the virtual lack
of internodes within the rosette of leaves.

The stalk of the Egyptian papyrus plant
(Cyperus papyrus) is a single elongate
internode.  The tassle at the top consists
of several dozen nodes packed closely
together through lack of any internode
elongation.

The growth of internodes is controlled by hormones, particularly gibberellin.  Dwarf forms of vegetables or other crops can be created by genetically limiting the plant's ability to produce or respond to gibberellin.  And plants that are normally compact, such as cabbages, can be stretched out like vines by applying the hormone to the young plants.

Gaillardia daisies are normally compact rosettes, but
under the influence of gibberellins can stretch out via
elongate internodes.

Internodes are therefore the unsung heroes of plant growth.  They determine whether a plant will be a skinny, stretched-out vine, a squat mound of leaves, or something in-between.

The first plants

Who were the first plants?   At the risk of a scolding from some of my fellow botanists, I’m going to define plants broadly as organisms that use sunlight to make carbohydrates, and release oxygen gas as a byproduct -   in other words, organisms that photosynthesize.   There is in fact a diverse array of organisms that photosynthesize, including some bacteria and various kinds of algae that are only remotely related to one another; not to mention photosynthetic sea slugs!  So used in that way, "plant" is more of an ecological term for organisms that provide the base of the global food chain, not a precise taxonomic category. (see my post of October 5, 2011, "Plants and animals and kleptoplasts - oh my!")

My goal today is in fact to explore where photosynthesis came from, and we won't be distracted by taxonomic issues.  The first organisms to practice photosynthesis were bacteria, and the cyanobacteria were the first to employ modern photosynthesis, in which oxygen is released as a byproduct.  Bacteria are prokaryotic organisms that have a simple cell structure without nucleus or internal organelles.  They don’t have chloroplasts like true plants or algae, but their entire cell is adapted to conduct photosynthesis.  In my October 5, 2011  post, I described how cyanobacteria in fact were captured and domesticated to become the first chloroplasts.   I also argued as devil’s advocate that cyanobacteria are the only true plants – the multicellular organisms that we call plants are merely the luxury condominiums in which those captured cyanobacteria live!

Cyanobacteria come in many different forms and resemble true eukaryotic algae.  Mat-forming cyanobacteria
 that build stromatolites are similar to to A and B, in which cells are bound by a mucilaginous matrix. on the
surface of a rock.  Drawing from Haupt, Plant Morphology, McGraw-Hill, 1953.

Photosynthesis was invented only once, and passed on to various eukaryotic organisms as chloroplasts, which have been captured, stolen, and recaptured many times.  But when did it all start?  We travel to Australia, where we find some important clues.

In a few shallow, highly saline lagoons along the west coast of Australia, peculiar knobby pillars of rock called stromatolites stand like the disarrayed remnants of a terracotta army, eroded and distorted beyond recognition.  These monoliths were not carved by some ancient civilization, however, but are built up very slowly by microscopic living organisms.   On the tops of the knobs, you can find mats of living microorganisms, held together by a mucilaginous glue.  The glue is secreted primarily by cyanobacteria.  Many bacteria form mats like this.  It’s a good way to anchor yourself to a suitable location.  

Stromatolites at Shark Bay, Western Australia.  Photo by Paul
Harrison via Wikipedia.

Cyanobacteria are massively abundant organisms.  Many live as free-floating plankton or tangles of filaments, and account for as much as 50% of the photosynthesis occurring in open waters (Fig. 2). Others are attached to rocks as filaments or mats.   They are easily confused with true algae, which are eukaryotic organisms with nuclei, chloroplasts, and other organelles.  Before electron microscopy showed us the difference, cyanobacteria were called “blue-green algae.”

Microbial mats on the tops of stromatolites form deposits that extend the knobby pillars slowly upwards.  Particles of sediment and lime precipitated from the water get trapped in the sticky matrix, and periodically bury the living microbes.   The resourceful cyanobacteria in those instances migrate to the top of the sediment and begin a new mat.  This results in a fine structure of alternating light and dark bands.  Stromatolites are thus, like coral reefs, built by the living organisms that inhabit them.   There is a website devoted to Shark Bay in Western Australia (www.sharkbay.org) that includes facts, photos and a video swim through a grove of stromatolites.

             
Stromatolites turn out to be one of our most important clues as to the origin of plant life.  They have been around for about 3.5 million years.  Ancient fossilized stromatolites, which can also be found in parts of Australia, are in fact among the earliest signs of any kind of life on this planet.  They are abundant throughout much of the geological record, but became rather scarce around 500 million years ago.  This was the time of the  “Cambrian Explosion”, when many new kinds of animals appeared.  Stromatolites came under attack by voracious grazing animals equipped with hard, scraping mouth parts.  After that, they survived only in restricted sites too salty for such animals. 

Though there is still debate about whether the earliest stromatolites were formed by cyanobacteria or some earlier form of life, or even by some physical process, there is no doubt that they were building stromatolites  by 2.7 billion years ago – still a heck of a long ago!   Fossilized cells identifiable as cyanobacteria have also been found in slightly different types of rocks, the Warrawoona and Apex cherts of Australia, and these appear also to date back to 3.5 billion years ago.   Cyanobacteria were the plants, the photosynthetic organisms, that supported the Earth’s early ecosystem.  They did so virtually alone form almost 2 billion years, after which the first signs of eukaryotic algae began to appear.

The early photosynthesizers must have gradually built up enormous populations, for the oxygen they produced eventually transformed the vast oceans of our planet, and then the very atmosphere itself.  The scarcity of oxidized (“rusted”) minerals in the Earth’s oldest rocks (older than 3 billion years), indicates that there was very little free oxygen in the atmosphere at that time, so the oxidizing of crustal rocks is also evidence of plant life.  Iron is particularly abundant on Earth, and quite prone to rusting.  In the ancient seas there was a steady supply of iron bubbling up from underwater volcanic fissures, and from eroding surface rocks.  In its unoxidized state, iron is soluble in water, but when it oxidizes it forms insoluble molecules of hematite or magnetite, which sink to the bottom of the sea.  When oxygen became available in comparably huge quantities there were spectacular depositions of iron.  This resulted in distinctive and extensive rock layers known as the Banded Iron Formations.  The “rusting of the earth,” as it was called by Schopf (2006), is the source of most of the iron ore that is being ravenously consumed by modern civilization.

There is some evidence of limited iron formations about 3.5 billion years ago, but they did not become truly massive until the mid-2-billions.   This suggests that oxygen buildup may have occurred sporadically and slowly at first, but became overwhelming between 2.7 and 2.5 billion years ago.  Oxygen makes up 21% of our current atmosphere, but 20 times as much plant-generated oxygen may be tied up in banded iron formations.  The formation of iron deposits declined rapidly after about 2 billion years ago, as the supply of dissolved iron was depleted, and oxygen then began to build up in the atmosphere.   The transformed atmosphere made it possible for more complex organisms to evolve, leading to eukaryotic cells and the modern world of algae, plants, fungi and animals.

So cyanobacteria invented photosynthesis as we know it today, and were the first functional plants.  The story does not end there.  In the two billion years of their unchallenged domination of the Earth, cyanobacteria also invented the rudiments of aerobic respiration and nitrogen-fixation, two other essential metabolic processes - cyanobacteria had to do virtually everything themselves!  Aerobic respiration is the process by  which organisms “burn” carbohydrates to fuel their metabolic processes.  It is obviously essential  for animals, but also for plants.  How else could they  utilize the carbohydrate reserves that they produced for themselves?

In terms of starting ingredients, photosynthesis is easy.  All you  need are carbon dioxide and water, and both are abundant on the planet.  But to make certain other things, like protein, DNA, ATP and other vital organic molecules, you also need nitrogen, which isn’t so easy to come by.  Wait-a-minute, isn’t the Earth’s atmosphere about 70% nitrogen?  Yes, it is, but it’s nitrogen that is hard to use.  Nitrogen gas consists of two nitrogen atoms bound tightly together by a triple bond.  Most organisms can’t break those bonds to make use of the abundant nitrogen supply.  Enter the cyanobacteria.  Somewhere along the way they acquired the genes to split nitrogen, either through mutation and natural selection, or from some other bacterium that invented it first.   Bacteria freely share genes through the process of horizontal gene transfer, a process we exploit in genetic engineering.

Once the nitrogen molecule has been split, cyanobacteria are able to attach hydrogen atoms to the nitrogen, making ammonia, and ammonia then can become the amino group required to make the building blocks of protein – amino acids.  So cyanobacteria, arguably the most successful and certainly the most long-lived group of organisms, make carbohydrates, metabolize carbohydrates for energy, and make their own protein.  The rest of us have only stolen from them!

References:

Schopf, J. W. 2006.  Fossil evidence of Archaean life.  Phil. Trans. R. Soc. B 361: 869–885.

Website for images and more information about stromatolites in Australia: www.sharkbay.org  (under "Nature of Shark Bay")