Tuesday, November 22, 2016

The Life of a Tree: 1. Integration--and the lack of it

Humans are very much animals.  We think like animals, and we have the most regard for the other animals that are our closer relatives.  Plants are so different from us that they're hard to understand.  That very foreignness may be one reason I like them. 

Let's contrast them. 

Yes, insects like these milkweed tussock moth caterpillars are animals.
(Ask yourself: does it eat? then it's likely an animal.  Most also move.)

Most animals move.  Animals have sophisticated senses: chemical sensors (the nose), light and image sensors (eyes), vibration sensors (ears), etc.  Animals--at least, those much more complicated than jellies, sponges, flatworms, and clams--have nervous systems that provide very sophisticated and centralized coordination and control.  They circulate blood in a loop-like circulatory system, they have nerves with a brain that receives and processes information and responds back.  A sensory stimulus to any part of the body might result in a response by any other part or by the entire body.

Ponkapoag Pond, Blue Hills Reservation, August 2016.

Plants have none of this.  They do not generally move, and need not seek food since they make their own using light energy; indeed, photosynthesis is one of the characteristic traits of the plant kingdom.  They have two more-or-less independent one-way transport systems: one (called xylem) transports water and nutrients from the roots toward the stems and leaves where they are important to photosynthesis; another (called phloem) transports the food (sugars and the like) made in photosynthesis to tissues that cannot make their own, such as the stem and roots. 

Plants respond to stimuli, also.  They can sense gravity: roots grow reliably down, shoots upward.  Plants are sensitive to light, and in particular use the length of darkness to help regulate seasonal changes.  Temperature is a stimulus that, for example, triggers the beginning of spring growth.  Plants are sensitive to various chemicals, and can (famously) respond to the chemicals produced by nearby plants under insect attack: they will increase their own production of defensive toxic chemicals.  But there is no nervous system and very little centralization; responses are decentralized and local. 

A tree I remember on my college campus had grown up and enveloped a streetlight; as fall lengthened into winter, all the leaves turned and fell except those in a sphere around the light: these remained green as they became increasingly tattered and finally died in the winter without turning--all because those leaves did not experience the long darkness that signals fall to a tree.

Trees seem to behave in coordinated ways in sending roots out into more moist or fertile soils, or increasing root or shoot growth in a way that balances the two.  But these are very simple and mechanistic responses: roots in richer soil have more resources and so will grow faster, while a cut tree stump will often sprout rapidly because it has all the resources of a disproportionately large root system supplying water and nutrients.  The result only looks purposeful

Plant vascular tissue.  Xylem--a dead tissue that resembles household plumbing
--transports water and minerals from roots to leaves, drawn upward by evaporation of water
from leaf cell surfaces.  Phloem, made of living cells, transports sugar by bulk flow
from where it is most concentrated (in summer, the leaves where it is created) to where it is
least concentrated (stems and roots that need food but cannot make it as green leaves can).

No comments:

Post a Comment