Control and CoordinationIntroduction
In the last chapter you studied the life processes that keep a living thing running. We began that chapter with an idea everyone has: if something moves, it is alive. Look around Aman's courtyard this evening. A cat streaks after a mouse, his little sister swings higher and higher, the buffalo chews and chews, and in a clay pot the bean seeds he planted last week have pushed the soil aside and come up.
All of this is movement, but not all of it is the same kind. Some movements happen only because something is growing: stop the growth and the movement stops too. Others have nothing to do with growth. Drag each movement into the right bucket.
A seedling moves only as fast as it grows; if its growth were stopped, it would never push the soil aside. The cat, the swing and the buffalo are different: nothing has to grow for them to move. Keep the touch-me-not in mind. It is a plant, yet its movement involves no growth at all. We will come back to it.
Every Change Has Its Answer
Why do we connect movement with being alive? One answer is that a movement is usually a response to some change in the surroundings. The cat runs because it has seen a mouse. Living things also move to use such changes to their advantage: plants grow out into the sunshine, children swing because it is fun, and a buffalo chews cud to break up tough grass so that it can digest it better.
Each change below calls for its own movement. Join every change to the response it triggers.
Notice how carefully every response fits its trigger. In class you whisper; you do not shout. So the body must first recognise what has changed, and then make only the right movement in reply. That needs a system of control and coordination. In multicellular organisms, specialised tissues do this job. This chapter is about how they work: first in animals, then in plants, which manage without any nerves or muscles at all.