132 ELEMENTARY BOTANY 
CHAPTER XX 
GROWTH - 
We have seen in the previous chapters that a plant breathes 
and takes in food material, including water, from the air and 
the soil. Out of the food material thus taken in, the plant 
builds up its own protoplasm, which is present in every living 
cell. There is thus constant chemical change taking place in 
a plant. Unfortunately it is impossible in this book to give 
the experiments by which this may be demonstrated, as they 
are very complicated, and need much apparatus; but from . 
what has already been said it will be clear that the chemical 
processes going on in a plant may be grouped under two 
heads : | | 
1. There is the building up ot complex substances from 
simpler ones. 
2. The breaking down of complex substances into simpler 
ones. 
The making of starch, the building up of the cell-wali and 
of protoplasm—the most complex substance in the plant—are 
instances of the building up of complex substances common to — 
all green plants. To these may be added the building up 
of special substances by certain plants, ¢., asparagin by 
Asparagus, olive-oil by the Olive plant; resin, gum, etc. 
The breaking up of protoplasm which is going on in living 
cells, and the consequent giving off of carbon dioxide, which is 
given out in respiration, is the most conspicuous instance of 
the second group. | 
When the building up of substances exceeds the heen 
down, or, in other words, when the income of a plant—that 
is, the food taken in—is greater than its expenditure, then 
_ growth takes place. ~ 
