
THE FRUIT AND THE SEED 295 
For simplicity we will mention only a few of the many 
known enzymes. Diéastase converts starch into sugar, so that 
the action of this substance changes an insoluble substance into 
a soluble one (Fig. 299). Jnvertase changes cane sugar into 
glucose and fructose. Lipase breaks up fats into their components, 
glycerin and fatty acids. Papain splits proteins into amino acids. 

Fig. 299. Stages in digestion of starch grains of barley by diastase 
Enzymes are just as important in animals as in plants. The 
process of digestion could not be carried on in their absence. 
The chemical composition of enzymes is entirely unknown 
and they can be recognized only by their action; but since 
many of them have been prepared as dry powders, they may be 
regarded as chemical compounds the composition of which we 
may hope to know some day. 
GERMINATION OF SEED 
Definition. The development of the seed into a young plant 
is called germination. A seed is said to have germinated when 
the radicle and plumule have reached out of the seed coat, but 
germination is not complete until the seedling has become estab- 
lished and is independent of the food supply stored in the seed. 
Period of rest. Seeds usually undergo a period of rest before 
germinating. The length of this period varies greatly in dif- 
ferent species, and in some cases it is altogether lacking. In 
the mangrove-swamp species of the family Rhizophoraceae the 
seeds germinate without falling from the tree. The first sign 
of the germination of such seeds is the projection of the long, 
slender radicle from the fruit (Fig. 300). After the radicle has 
grown to a considerable length the seedling drops from the 
