How plants do everything without moving a muscle

So more about the difference between plants and animals. In my last post ("Why we must teach botany"), I indicated that photosynthesis dictates immobility, and that the inability to move has ramifications for all other aspects of plant life.  One of the challenges of teaching botany is to convince students that organisms that don’t look or behave like their pet hamster are in fact far more interesting!  And as it turns out, our favorite things about plants, from flowers to flavorings, only exist because plants can't move.

Plants must spread their tissues thinly in light-
gathering antennas known as leaves, with an
equivalent underground system of water and mineral-
gathering roots.  This architecture makes
movement impossible.  Pictured is a species of
Tetrapanax from China.

The word “animal” means something that moves, as in the word “animate.”  Why don't plants also move?  Wouldn't it be useful for them to pick up and move to  a better lit or more fertile location, or to swat away annoying herbivorous insects?  And how do plants have sex if they can't move?  It all begins with how animals and plants feed themselves.

Animal food comes in the form of concentrated, nutrient-dense packages: other organisms. Animals move in order to locate, go after, capture, and consume these discrete food items.  Secondarily, they move to reproduce, escape danger, or defend themselves.  These activities require muscles, nerves, senses, a mouth and an internal digestive system.  When animals became multicellular, their cells wrapped compactly around the central digestive cavity, and one end became specialized for feeding and coordinating movement. An animal body is therefore discrete, streamlined, and highly organized, with a distinct head for sensing and biting, fixed locomotory organs along the sides, and, in most, a rear end for excreting wastes.  

   

The resources needed by plants, on the other hand, are diffuse: carbon dioxide, light, water, and dissolved minerals.  In order to efficiently gather these far-flung molecules and photons, plants must spread their tissues broadly and thinly, and behave more like antenna systems than hunters; the more they spread, the more resources they can gather. So inherent to the plant lifestyle is indeterminate growth - the ability to grow, branch, and expand their antenna systems indefinitely.

Some single-celled algae move about in response to light
intensity, but this is impractical for multicellular plants, as
simple flagella would have to be replaced by muscles, a
 coordinating nervous system, and a streamlined body ill-
suited to photosynthesis.

Single-celled, flagellate algae like dinoflagellates, euglenoids, and some green algae do move in response to light and other stimuli, but as plant life became multicellular, the costs of motility apparently outweighed the benefits.  How would you push a satellite dish through the water, or march a tree across a savanna?  The shape requirements for motility and for photosynthesis are architectural opposites, but for every reason animals have found movement necessary, plants have found alternate strategies. 

The famous photosynthetic sea slug, Elysia clarki, would seem
to be an exceptional animate "plant."  Sea slugs must still
obtain their chloroplasts by ingesting algae, and so still need
the full motility apparatus of animals. If a sea slug were able to
pass its chloroplasts onto its offspring and never have to eat
again, its descendants would most likely gradually lose their
ability to move as they expanded their photosynthetic surface
further, and perhaps even take up a plant-like habit of branching
and indeterminate growth.

If a plant becomes shaded, for example, it can usually branch out and extend into a light patch.  A vine thus finds its niche in a dense forest.  Roots likewise can extend toward moister or more fertile patches of soil.   So indeterminate growth can relocate antenna systems as well as expand them, making motility not only difficult, but also unnecessary.   

The plant body is what it needs to be for efficient photosynthesis, but without moving parts, how do plants, particularly terrestrial plants, do anything else?  How do they circulate water and food, for example?

Animals circulate food, water, and wastes through a muscle-driven circulatory system. Lacking that, plants have developed a simple passive system that operates pretty much like a paper towel.  The walls of plant cells are made of cellulose, which by no coincidence is the material paper towels are made of.  Water moves upward into a paper towel, even defying gravity, because of the magnetic attractions among water molecules and cellulose.  The plant is a giant, complex paper towel sopping up water from the soil. Evaporation of water at the top of the plant pulls more water up from the roots, keeping the entire system saturated and moving (see How does water get to the top of a redwood tree?). This is transpiration, which requires no energy expenditure on the part of the plant and no moving parts. 

For other activities, plants exploit something called turgor pressure, which in turn is the result of the enigmatic process of osmosis.  The short description of osmosis is that water tends to move into cells, causing them to expand.  If that works for you, you can skip to the next paragraph.  The explanation for why this happens is more complicated.  The net movement of water across a cell membrane is from areas of higher water concentration (e.g. distilled water) into areas of lower water concentration (i.e. a solution of other molecules), even though it would seem that the latter area is already “crowded.”  Water molecules move randomly in both directions, but since there are more of them outside the cell than inside, the number entering at any moment is greater than the number that are exiting.  The other molecules in the cell cannot exit as easily, so the cell just gets more crowded.  As the total number of molecules bumping around inside the cell increases,  the pressure increases.  Incidentally, the opposite happens when you put a cell into salt water – water leaves the cell and it shrivels.

The leaves of the venus fly trap snap shut around prey by
manipulating osmotic water pressure. Water pressure
in cells (turgor pressure) drives their expansion
during plant growth, and also maintains the crispness
and upright posture of soft plant parts.  It also
moves dissolved food around the plant through the phloem.
Water tension resulting from transpiration pulls water
upward, sometimes 100 meters or more.  All this happens
without muscles or nervous system. 

 A naked animal cell in fresh water will expand until it bursts.  Plant cells, however, are surrounded by rigid cellulose walls that do not allow the cells to expand.  So the pressure builds up to the point at which it starts forcing water molecules out of the cell at the same rate they enter through osmosis.  The pressure then stabilizes and we know it as turgor pressure.

So healthy plant cells are pressurized, and this creates a force that plants exploit in a variety of ways, replacing the force of muscular activity in animals. Turgor pressure is what keeps soft plants upright, and lettuce crispy.  It causes young cells with soft walls to expand during growth of shoots and roots.  It is also the basis for venus flytrap leaves snapping together, through a sudden decrease in turgor pressure.  In the phloem tissue, sugar is actively pumped into specialized cells, resulting in even more osmotic pressure.  These cells are connected in long tubes and the pressurized fluid flows to areas of lower pressure, where sugar is being actively removed.  Thus sugar may flow from leaves to roots, or from roots to new shoots, flowers, or fruits.

How do plants reproduce while stuck in one spot?  The answer, my friends, is blowing in the wind (with apologies to Bob Dylan!). Sperm cells can move only short distances and must remain wet.  Sexual reproduction, on land at least, can therefore occur only between individuals that are literally touching one another. To be worth the trouble, however, sperm cells must unite with eggs from a genetically different individual.  While animals run around in frantic hormone-driven pursuit of a mate, plants have a sublimely simpler solution: they release tiny airborne spores to serve as genetic couriers. 

When we see a fern in the forest, we're looking at the
sporophyte generation, which produces spores asexually.

The gametophyte generation of ferns
is a small, independent, short-lived green
plant.  Sexual reproduction occurs
between adjacent gametophytes.  The
resulting fertilized egg will develop
 into another full-sized fern
 (the sporophyte).

Ferns, for example, produce thousands of tiny spores that swirl around in the breeze, and which with luck land next to spores produced by genetically different individuals.   Each spore grows into a tiny, specialized, short-lived plant, and it is these tiny plants, the gametophytes, that undergo sexual reproduction.  Sperm cells from one swim the short distance to the eggs produced by another.  (The full-sized plants that produce the spores are called sporophytes). The life cycle of a plant thus consists of the alternation of  full-sized individuals with tiny ones. (See “the truth about sex in plants.”)

In flowers, such as this orchid in the genus Cypripedium,
the gametophyte generation is hidden within seeds
and pollen grains. The elaborate structures of flowers
are devices for luring insects and
other animals to carry sperm-containing pollen grains
from one immobile plant to another.

The gametophytes of seed plants are even tinier than those of ferns and hard to see.  A pollen grain is a fancy spore that moves sometimes great distances and carries within it the tiny 3-celled sperm-producing gametophyte.  The egg-producing gametophyte is likewise hidden away inside an embryonic seed (ovule).  So when a pollen grain is delivered to a pine cone or a flower by wind or insect, it brings sperm cells directly to the eggs. Plants can thus reproduce with genetically different individuals that may be miles or hundreds of miles away, and they don’t have to move an inch.

Finally, without the ability to run, hide, or bite, how do plants defend themselves against the hoard of vegetation-munching insects, grazing mammals, and other herbivores? They have a variety of tricks, but primarily what they do is to make themselves toxic or at least distasteful to most animals.  Plant are master chemists, and have continually come up with new poisons to counter the ever diversifying array of vegetarians.  Those poisons, incidentally, are the sources of our medicines, drugs and spices, as well as overt poisons.  The difference between a medicine, or even a spice, and a poison is a matter of dosage.

Spices, such as the yellow curcumin from the rhizome of
turmeric plants (Curcuma longa), are chemical defenses against herbivores,
as are other substances we use as drugs, medicines, poisons, resins, etc.
Photo by Simon A. Eugster, posted and licensed on Wikimedia Commons,

Plants can also create physical barriers that deter animals from even taking a bite: layers of dense fibers or bone-like sclereids within their tissues, or spiky thorns, spines, and prickles on the surface.  Grasses and some other monocots invest relatively little in chemicals, but regrow rapidly from their leaf bases after an attack of herbivores.  

Plants thus achieve all of life's basic functions without moving a muscle, and flourish in great variety and numbers.  A lion can growl and bite and claw its way through an existence of a couple of decades, but a tree can live for 5000 years.

Why we must teach botany

Those of you who follow this blog site regularly may be wondering where I've been for the past several months.  Aside from teaching this semester and having to move my office, I've been finalizing the manuscript of a new book: “Plant Life – A Brief History,” which will be published this winter by Oxford University Press.  My reason for writing this book is the same as for doing this blog site – to explore the mysteries of why plants are the way they are, and to help people around the world to better understand this fascinating and important group of organisms. 

I hope my efforts will also contribute to the Botanical Society of America’s fight against “plant blindness,” and its campaign to “reclaim the name” of botany. (go to  http://www.botany.org/) So what’s all that about?  There has been a serious decline in the teaching of botany in university biology departments over the past 40 years.  The  reasons for that are complex, but primarily the result of the burgeoning growth of cellular, genetic, and developmental biology, as well as advances in theoretical ecology, that have come to occupy an increasing share of an undergraduate’s budget of course requirements.  This is on top of the traditional bias that plants are less interesting or less important than animals.

Like fine arts and PE in elementary schools, botany and other taxonomically defined classes are viewed as luxuries that can be cut if needed – and they have been. In some universities, even the introductory botany course has fallen by the wayside, leaving only a week of plants in an introductory biology sequence.  Even that is typically taught reluctantly by a young (or old) faculty member who is only one chapter ahead of the students in his or her own removal from plant blindness.  

The excuse is often given that “most of our undergraduates these days are pre-med.” That doesn’t excuse a department from providing a well-rounded background for those who plan to go into research or teaching.  Moreover, when courses in medical botany are offered, they are very popular, and provide to our future doctors, nurses, etc., a vital broadening of perspective on the nature of medicine.

Plant blindness then refers to a general lack of awareness of plants, particularly of their diversity and the many unique ways they contribute to the balance of the natural world, as well as to our own societal and individual well-being.  It is akin to other forms of blindness, like the much deplored inability of young people to point out Iraq, or even Texas, on a map.  Our undergraduates, graduate students, applicants for our vacant positions, and ultimately our fellow faculty members increasingly have less working knowledge of plants.

There’s no question that this is bad at all levels.  At the general level, failure to understand both the fundamental biology of plants, and the significance of plant diversity, leads to superficial and erroneous interpretation of environmental issues, abuse of our food supply chain, nutritional and medicinal resources, poorly designed and maintained landscapes, and numerous other issues vital to our survival.   K-12 teachers turned out by universities without a good background in plants will reinforce and amplify the blindness. 

At the professional level, biologists may overlook cellular, genetic, and developmental processes, or environmental adaptations, unique to plants.  Consideration of how plants do things can provide a breakthrough when animal or microbial models hit a dead end.  A great comedian draws upon his or her own accumulation of diverse observations of culture and human behavior to synthesize a unique and entertaining routine.  A master medical diagnostician draws upon a vast pool of specific information about symptom and their causes, to identify and cure an ill patient.  And so too, the most brilliant and imaginative biological researcher will draw upon his or her knowledge of diverse organisms to ask new questions, or to find different solutions to existing questions. 

We must of course teach critical thinking and the scientific method in science, but there must also be a place for teaching awareness of diversity, and providing opportunities to build a pool of knowledge about different organisms. 

In addition, the lack of awareness of plant diversity and the fact that every species interacts uniquely, whether subtly or dramatically, with its environment can lead to serious errors.  It can lead researchers to oversimplify the role of plants in ecosystems, or worse to fail to obtain accurate identifications of the plants studied (see Bortolus 2008, for some classic blunders).  Misidentification links the study plants, as well as the results of the study, to the wrong body of literature.

         

The “reclaim the name” movement reflects the parallel decline of respect for botany among our biological colleagues, and the impression that botany as a subject matter is old-fashioned and no longer important.  Will a name change help?  It seems that everything in our society gets renamed every few years in order to buy new respect.  Used cars are now “pre-owned” and I’m not sure what graveyards are called these days.  Some of my colleagues insist that we call our discipline “Plant Science,” but to me that has an applied, agricultural ring to it.  I haven’t seen any of my zoologist friends opt for “Animal Science,” which if I remember correctly from my years at Cornell is the study of dairy cows! 

The counterpart to zoology is “phytology,” which has never caught on as the name of our discipline.  If we ditch botany, what do we do with botanical gardens?  “Plant gardens” just sounds dumb, and people would just drive past a ”phytarium,” having no idea what the heck it was.  

I, for one, am a BOTANIST.

The term botany has indeed evolved into something that encompasses the big picture of plant life, of the unique attributes that unite plants, as well as of the multiplicity of unique ways in which plants have adapted for survival.  This is what distinguishes those of us who call ourselves botanists from cell biologists, geneticists, etc. who happen to be working with plant models at present.  Being a botanist, however, is not a research specialty, but a label that can be appended to any researcher who has had a broad training in botany, and/or a sufficient interest in the big picture to self-educate.  

That broad training – a full curriculum in plant anatomy, morphology, physiology and taxonomy, is harder to find these days, but does survive in departments affiliated with agricultural colleges and a few other refuges of enlightenment.  Let’s hope they continue the tradition!

So outside of those botanical monasteries, what do we do with our 15 minutes in the spotlight?  What are the essentials of botany that every undergraduate should have?  Earlier, I posted the “Essential features of plants.”  Those, at least, we need to impart in our week in introductory biology.  We need to avoid getting bogged down at that level in too much descriptive details, but perhaps demonstrate, with some well-chosen examples,  how different plants can provide radically different solutions to the same problems of survival.

If we are able to provide a full semester introduction to botany, we should do so aimed at a broad audience of science majors, from biology to geology and anthropology, with minimal prerequisites.  Beyond that, botanists in our faculty can craft more specialized courses based on their own background and experience. 

The introductory course, beyond the essentials, will have an emphasis on evolution,  diversity and ecology.  Discovery and explanation of adaptive differences among plants is what makes botany so exhilarating, and what can come as startling revelations to the plant blind.  Again, we must pull back a bit from the details that fill textbooks designed for botany majors. 

I leave you with a simple comparison of plants and animals, which borrows from the biblical model of “begats.”  A framework like this can serve as the starting point for a lecture, a class, or an entire botanical curriculum.

For plants:

  1.       In the beginning was photosynthesis.

  2.       Photosynthesis begat indeterminate growth.

  3.       Indeterminate growth begat immobility.

  4.       Immobility begat hydrostatic engines, spores, and passive defense.

  5.       Hydrostatic engines begat maple syrup, fresh salad, bamboo, and venus fly traps.

  6.     Spores begat alternation of generations, pine cones and orchids.

  7.       Passive defense begat curry, digitalis and marijuana, as well as cactus spines and walnut shells.

The resources required for photosynthesis are diffuse:
light and carbon dioxide from above, and water and
minerals from below.  How does that dictate virtually
every aspect of plant biology?

For animals:

  1.      In the beginning was the mouth.

  2.       The mouth begat food-sensing organs and locomotion.

  3.       Food-sensing organs and locomotion begat the head.

  4.       The head begat response, behavior, instinct, sex, thought, and blog postings.

Finer iterations of this model can lead us to understand the difference between Acacia trees and savanna grasses, both struggling to survive in an African savanna, or the difference between mussels and barnacles vying for a spot on an intertidal rock.

Your homework is to think about these chains of cause and effect.  How does photosynthesis lead to curry?  I’ll return with a fuller discussion in the near future.

Literature cited:

Bortolus, A.  2008.  Error Cascades in the Biological Sciences: The Unwanted Consequences of Using Bad Taxonomy in Ecology.  Ambio 37 (2): 114-118.