Tuesday, November 29, 2016

The Life of a Tree: 3. Growth

Most animals are integrated in another important way: their growth.  You (as a representative animal) grew from the cell that resulted from conception in a highly organized dance in which cells actually moved from place to place, divided (at different rates in different places and times), and differentiated into all the many tissues and organs that make up the adult human.  (Just how all of this is organized and regulated is cutting-edge science today.)  Then, once you had reached adult size, your cells stopped dividing.  Just stopped.  --with the exception, of course, of those needed to replace skin, blood cells and the like, and those triggered to divide in wound healing, and so on.  

Plants do this is a very different and simpler way.  Plant cells are encased in a cell wall that strengthens them, provides overall structure to the plant, and effectively fixes the individual cells in place--and is one reason plants don't move.  Instead of growing everywhere, land plants grow at shoot and root tips, at regions called meristems.  Cells in a meristems divide, then those that end up on the side opposite the tip differentiate into vascular tissue, fibrous tissue, cortex "filler," leaf primordia, etc., while those that are nearer the tip continue to divide, gradually leaving the differentiated stem or root cells behind as the shoot elongates.  

 Shoot (Coleus?) and root (onion) meristems.  Tissue slices have 
been stained to show individual cells and their nuclei more clearly. 

One consequence of apical growth that surprises people is that the tree branch you have to duck under this year will never be any higher: its height was established when that branch was only a twig.  (Those lower branches tend to die over time, though, often leaving the lower trunk bare.)   

Unlike animals, most plants have no "adult size": as long as they live, trees and shrubs continue to grow.  Because each year's shoot growth begins with last year's buds, to cease to grow is death.

Besides growing at tips, woody plants (trees, shrubs and vines) also grow around their circumference.  A layer of tissue under the bark, a lateral meristem called vascular cambium, adds cells inward to form water-carrying xylem tissue, and outward to form sugar-carrying phloem tissue.  (When you look at a piece of wood, you are looking at xylem, and the tiny holes sometimes visible in end grain are the cut ends of water-carrying tubes called xylem elements.)  While xylem is long-lasting, typically carrying water for many years before finally clogging up and becoming dark-colored "heartwood", phloem is only active for a short period, eventually becoming a second kind of lateral meristem: bark cambium.

This is a good place to talk about the chemistry of animal and plant strength.  We mobile animals are a peculiar mixture of delicacy and strength: our individual cells are floppy, insubstantial little blobs of Jello, but together they secrete proteins that form immensely strong fibers (mostly collagen) that make our bodies tough and strong and yet flexible.  The walls of plant cells are made of a very different fiber called cellulose, which is a polymer of sugar molecules instead of a protein.  

 Loose cells are easily scraped off the inside of your cheek with a dull toothpick.  
These are stained to show the nuclei.  Notice how floppy they are!

Cells in the leaf tissue above have been soaked in a salt solution so they have wilted.
The living tissue of each cell (complete with green, disc-shaped, sugar-generating chloroplasts)
have collapsed, leaving the box-like cell walls intact.  Think of severely wilted lettuce.

 A cross-section through a stem shows thinner-walled cortex cells with thicker-walled cells.  Producing linen begins with beating stems of flax to separate these strong vascular fibers
from the rest and spinning them into thread. 

The little cellulose boxes in which plant cells live make them individually tough even as they trap the cells in place.  (In fact, green plant cells are hydraulic structures: their rigidity is due to internal water pressure in exactly the same way a football's rigidity is due to air pressure.)*  Animals must continually bathe their cells in a mild salt solution that prevents them from either shriveling up (too much salt) or exploding (too little).  This vulnerability is the reason athletes must watch their electrolyte (salt) balance.  Though plant cells may wilt with too little water (or too much salt) , these cells are immune from damage by fresh water because their cell walls are strong enough to prevent their bursting.  Together, these cell walls form the fiber of countless natural products, from the cotton in our clothes, to the rope that formed the rigging of tall ships.  In a form stiffened by other molecules (lignin prominent among them), this fiber becomes the wood that builds our homes.  Wood, sometimes disparaged in comparison with modern materials, remains stronger for its weight than any other substance. 

*I remember the first time I handled a deflated football: the pigskin is soft and flexible (like a plant cell wall).  Inside it is a delicate but air-tight rubber bladder (like a cell membrane).  The pigskin prevents the inflated bladder bursting just the way a cell wall prevents the plant cell membrane from bursting.  

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