THE PLANT AND ITS FOOD § 15 
secreted over the surface of the living sub- 
stance, which is henceforth shut off from the 
outer world throughout the vegetative life of 
the individual. It is usually in connection 
with certain reproductive processes only, as, 
for example, when a new generation is about 
to arise, that the plant-protoplasm is more 
or less freed from the cellulose skin with 
which it is almost invariably clothed at other 
periods of its existence. 
The simplest method of realising what all 
this means to a plant is to study some definite 
example, when other salient features of 
plant life will also come directly under notice. 
For this purpose one of the common lowly 
plants belonging to the algw may be chosen, 
and we will select as an example a microscopic 
organism belonging to the genus Chlamydo- 
monas. : 
This plant is of fairly common occurrence 
in ditches and pools, especially in late spring 
and -in the autumn. Its body consists of 
a single cell, that is to say its somewhat 
oval-shaped protoplasm is contained within 
a single membranous cavity. At one end 
two vibratile hair-like filaments of protoplasm, 
called cilia, project through the membrane, 
and it is by means of these that the little 
plant is able to swim actively through the 
water in which it lives. Within the proto- 
plasm of the body, and just beneath the spot 
where the cilia sprout from it, are two con- 
tractile vacuoles—hollow spaces filled with 
