Monday, November 28, 2011

How does water get to the top of a redwood tree?

The upward movement of water more than 100 meters in a redwood or eucalyptus (there is  a traditional dispute between Americans and Australians over who has the tallest trees!) seems to be a gravity-defying task of epic proportions.  Gravity is certainly a factor, and ultimately limits how tall a tree can get, but there are other forces at work that can meet that challenge.  The amazing thing about plants is that the process is largely passive, in the sense that plants expend practically no energy to accomplish it.  There are no muscles and no heart in a tree to pump water upward.  What there is, is basically a gigantic paper towel.

You may recall the ads for the “Bounty Quicker-Picker-Upper”  a decade or two ago.  In the ads, these paper towels quickly absorbed any spilled liquid.  You can take a paper towel (even a cheap slower-picker-upper), roll it into a tube, and stick the end in water. You’ll quickly see the water begin creeping up the paper towel.  We see the same gravity-defying process in plants.  What is a paper towel made of?  Wood!  And the magic ingredient in both a paper towel and a living plant is cellulose. 

Every plant cell is wrapped in a layer of cellulose – the cell wall.   In wood, the cells of the xylem die after laying down strong cellulose walls, leaving the latter as narrow, empty conducting tubes.  These tubes line up so as create masses of interconnected passageways through which water can move freely.  So a tree trunk is essentially a massive, non-living, interwoven paper towel.   

But what is the interaction between cellulose and water?  What is the force that can overcome gravity?  The short answer is magnetism.

Magnets can defy gravity by lifting nails and other iron objects.  No, neither cellulose nor water is made of iron, but the forces involved are similar.  A molecule of water is electrically charged.  Imagine each molecule as a Mickey Mouse head.  The face is an atom of oxygen, and the ears a pair of hydrogen atoms.   The two types of atoms are bound together by sharing electrons, but the oxygen atom holds onto the electrons much more tightly than the hydrogen atoms do.  the result is that the electrons hang out around the oxygen atom most of the time, giving its side of the molecule a net negative charge.  The two hydrogen atoms are left with a net positive charge as they are visited less often by the electrons.   Thus water molecules are thus like tiny magnets and tend to stick together.  This actually is what makes water liquid, rather than a gas, at room temperature, and which accounts for a lot of other properties that we don’t have time to review here.

It so happens that the complex surface of cellulose fibers also have positive and negative charges, and so attracts water molecules.  In a paper towel, water molecules are pulled into every available niche in the cellulose matrix.  Those at the top are pulled into higher niches, which pulls more water molecules from the bottom.  Narrow spaces within the matrix also fill with water molecules, which attract and pull each other in.  This happens until the towel is saturated and there is no room for any more water. 
The water molecule (A) consists of a negatively charged oxygen atom and two positively charged hydrogen atoms.  This causes them to stick together in chains (B) and to the walls of cellulose fibers (C).

If we return to our rolled up paper towel with the end sitting in the water, we can watch something else happening over time.  The paper towel only gets saturated at the base, and even remains fairly dry at the top.  But given enough time, the container of water it is standing in will completely dry up.  Of course what is happening is that the water that has moved up the column continually evaporates when exposed to the air, leaving spaces for more water molecules to move up.  The result is a steady stream of water moving upward, drawn by both evaporation and the electromagnetic attraction of water molecules to the cellulose and to each other. 

This is the process of transpiration, which is powerful enough to lift water and dissolved minerals to the top of a tall tree.   It continues as long as water is evaporating through the leaves.  In extremely humid weather, or if the leaf pores (stomata) shut down for the night, the stream is suspended in place until evaporation resumes again later. 

I should note that a tall tree, if dried out, cannot start this process from scratch.  Gravity will stop the electromagnetic movement of water long before it can reach the evaporation zone of the leaves.  The transpiration stream develops in a germinating tree seedling and is maintained and strengthened as the tree grows, but if that stream should be broken (interrupted by extensive air bubbles) in an extreme drought, it cannot be repaired and the tree will die.   Only small plants like mosses can completely dehydrate and recover when wet conditions return. 

Friday, November 18, 2011

What is a vegetable?

The morning headlines are telling us that Congress has declared pizza to be a vegetable.  Pundits around the world are reacting to this news from every conceivable point of view.  I will leave the nutritional arguments to nutritionists, the political arguments to the Occupy Wall Street movement, and making fun of congress to Jon Stewart.  So what is the botanical perspective?

First, one must note that congress did not actually declare pizza to be a vegetable. That was a revision made by the “media,” to make the story more sensational.  Congress was simply legislating that tomato sauce is a vegetable, and that the amount found in a pizza qualifies the latter as a balanced meal (something college students have always known).  Again it is basically a nutritional/political issue, but perhaps I can address the point of whether or not tomato sauce is a vegetable.

Botanically, there are widely varied definitions of the word “vegetable.” It is not an officially defined technical term.   The plant kingdom has at times been referred to as the “Vegetable Kingdom,” and if you’ve ever started out a round of 20 Questions by asking “animal, vegetable, or mineral,” you’ve employing that definition.  So by that definition, anything that comes from a plant is a vegetable, including wheat flour.  A pizza, if totally vegan, could therefore could be classified as a vegetable, along with donuts, chocolate chip cookies, and fruit loops.  Now if the members of congress had boned up on their botanical history, they could have used that definition and made this whole debate a lot simpler.

It is more useful to break down the different categories of plant parts, and consider their role in nutrition.  In terms of the parts we eat, we can consider the following classification:

1.       Leaves and stems – these are your hard-core vegetables, low in carbs, high in vitamins, minerals, and fiber.  This is what nutritionists are talking about with respect to improving the quality of school lunches.
2.       Seeds – these are the structures containing embryos and stored food for the next generation of plants.  They include peas and corn, which are considered “starchy vegetables,” along with various kinds of beans, which are sometimes considered vegetables, sometimes not.  Actually a kernel of corn (maize to the rest of the world) is botanically identical to wheat, oats, barley, etc., and we don’t consider popcorn, corn muffins, or fritos to be vegetables, so corn is marginally a vegetable at best.  The “baby corn” popular in oriental cooking works better, because it hasn’t stored up the starchy reserves yet. (technically, corn, wheat and other cereal grains are single-seeded fruits with a very thin, hard fruit wall – but that’s getting into botany geek territory and we don’t have to go there).
3.       Starchy roots and other underground structures.  Many plants store food underground for use in the next growing season.  They are loaded with starch, and so can contribute a lot of calories.  Carrots, beets, and radishes are true roots, as are sweet potatoes, but white potatoes are specialized underground stems called tubers.  Onions might have the best claim to be true vegetables, since they consist of compact cluster of specialized leaves, but they also store carbs and are calorie-rich. 
4.       Fruits – technically a fruit is a structure that encloses seeds in plants, but many fruits that are not sweet are used as vegetables: peppers, eggplant, okra, cucumbers, squash, green beans, and … tomatoes! So botanically speaking tomato sauce is a fruit product, but we yield to common usage on this point, because tomatoes are low in sugar, high in vitamins, lycopene, etc.  Tomato sauce can therefore be considered at least a vegetable product, along with ketchup. The problem with these is what else is in them – like up to 60% of your daily allowance of sodium in a cup of tomato sauce.

So, from a botanical perspective, you can use the word vegetable in almost any way you want in reference to the parts of a plant, and so it becomes irrelevant nutritionally unless carefully qualified.   Nutritionists should have the final say, because it’s what’s in those vegetables and vegetable products, and the overall balance between the different food groups that is the issue.

Wednesday, November 16, 2011

The truth about sex in plants

It may surprise you to know that plants reproduce by sperm and egg, the same as animals. Well not quite the same, of course, since plants can’t move around or physically interact during the process.    This was a huge problem for the first plants to creep up onto dry land.  Their ancestors were green algae, who as a group mostly release their sperm cells (and sometimes their eggs as well) directly into the water to fend for themselves.  In fact many aquatic animals including sea stars and most fish, follow the same strategy.    Releasing sperm and egg cells into dry air, however, just doesn’t work!   
The seed plants eventually solved the problem through the invention of pollen grains -  private limousines  in which  sperm cells ride comfortably through the air. Pollination of pinecones and flowers is what you usually hear presented as the sex life of plants.   But the technology of pollen grains was long in coming.  How did the first plants manage with their sperm cells flopping around on dry land?
Sperm cells were indeed naked and vulnerable as they made their first journeys on land, and they still are in some modern plants like mosses and ferns.   Mosses and ferns are, however, numerous and diverse.  In the sex department, they apparently do just fine, thank you very much.  But how exactly? We’ll focus on ferns for now.  Mosses do things a bit differently, and we’ll return to them in a future post. 
 Like their aquatic ancestors, the sperm cells of ferns must swim through water to find an egg.  This can only take place when the soil is quite wet, and even so, sperm cells can’t travel very far - a few centimeters at best, so suitable mates must be quite close.  However, even if sperm and egg cells were dropped from the fronds of adjacent ferns, any two plants within sperm range of each other would most likely be siblings or at best  cousins, (or at worst  branches of the same plant!) Reproduction could take place only between nearest neighbors, generation after generation.  Plants would exist in pockets of incestuous interbreeding, and everyone knows how bad that would be.  The whole evolutionary purpose of sex is to create genetically diverse populations through breeding among widespread and genetically different individuals.  How did the early land plants escape from this trap?
The solution is something called alternation of generations.  This involves separating the reproductive process into two phases, with a small alternate “individual” as a go-between.  So there is the main plant that we see every day, and a small temporary plant that lives just long enough to complete its reproductive function.   In the fern, the main plant produces not sperm and egg but spores.  These can be seen at certain times of the year, produced in dense clusters of spore chambers (sporangia) on the lower surfaces of the fronds.  The spores are hard and dry and can be dispersed great distances by the wind.  The spores of different individuals, sometimes from widely spaced populations can whirl around in the wind and by chance land together in a suitably moist piece of ground.  
The tiny gametophytes of ferns are
typically little more than 1 cm long
and lie flat against the ground, Sperm
and eggs are produced in tiny
on the undersurface, among with the
root-like rhizoids seen here. from
Haupt, A. W. 1953. Plant Morphology.
McGraw-Hill. New York.
These genetically mixed populations of spores then germinate into the tiny alternate plants that are small enough and close together enough for sperm cells to swim between them.  So it is these alternate plants (the gametophytes) that conduct the sexual phase of reproduction.    In fern gametophytes, sperm cells and eggs are typically produced at slightly different times, so as to avoid the dreaded self-fertilization.  In some species individual gametophytes produce either sperm or eggs, not both – an even better insurance against self-fertilization. 

The large, ordinary plants that we
recognize as ferns are the
sporophytes.  Spores are produced on the
undersides of the fronds.

The big fern plants (the ones we actually see when we walk through the woods) are called sporophytes, because their reproductive task is to produce spores.   Ferns sporophytes have two sets of chromosomes in every cell (they are diploid) like animals, and the special cells on their fronds that produce spores undergo meiosis, a special type of cell division that results in cells with just one set of chromosomes (they are haploid).  So the spores, the gametophytes, and the gametes are all haploid.  When sperm and egg unite, they produce a diploid zygote, which develops into a new diploid sporophyte.   In ferns and seed plants, it is the sporophytes that get large, living often for many years, while the gametophytes are very small and ephemeral.  In mosses and liverworts that situation is reversed, but I’ll get to that in another post.
By separating the reproductive process into two phases, or “generations,” plants overcome their own immobility as well as the limited mobility of their sperm cells, and produce genetically diverse descendants.   Long-distance dispersal and mixing of different genotypes is accomplished by inert spores, and the joining of sperm and egg is then accomplished over a short distance by the tiny gametophytes.   Animals that move about and choose their mates don’t need an intermediary gametophyte.  They produce sperm or egg directly through meiosis, and the males actively deliver the sperm to the females themselves.
But how does all this translate into pollen grains?  Seed plants still employ tiny gametophytes for producing sperm and egg, but these are even tinier and harder to see.  To make a long story short, in the transition from fern-like ancestors to the first seed plants, specialized, sperm-producing gametophytes shrank down into tiny 2- or 3-celled structures that remained within the wall of the spore.   In other words, a pollen grain is a spore that contains a tiny, prepackaged male gametophyte.  The female gametophyte, containing an egg, forms within the embryonic  seed (ovule).  The pollen grain, like spores in general, is dispersed some distance from its parent.  When it lands in the vicinity of an ovule on another plant, a pollen tube develops that carries the sperm cells directly to the egg.   Neither the sperm-producing gametophyte nor the egg-producing gametophyte lives on the ground, and the sperm cells don’t have to swim through wet soil.   This allows seed plants to live and reproduce in a greater variety of habitats than their predecessors.