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