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.