THE PLANT AND ITS FOOD 17 
liquid which are continually contracting and 
expanding.» This kind of vacuole is also 
characteristic of many of the _ primitive 
animals, whilst it is only met with in com- 
paratively few of the more primitive and still 
motile plants. A small, somewhat refractive, 
spot in the protoplasm marks the position 
of the nucleus, an important structure which 
is found in the protoplasm of animals and 
plants alike. Another part of the protoplasm 
is coloured green, and is clearly defined from 
the rest of the living substance not only by 
its colour, but by its denser consistency. It 
is termed the chloroplast, and it often contains 
a clear spot in its interior, termed the pyrenoid. 
Finally, close to the point of insertion of the 
two cilia, there is a small brown or yellow 
‘““ eye-spot.”” The little plant swims about 
through the water, and though the movements 
appear at first sight to be aimless, they are not 
altogether so, for if a large number of in- 
dividuals are present, so as to give the water 
a green tinge, it ig seen that they congregate 
on the illuminated side of the vessel. That 
is to say, they are affected by the stimulus of 
light, and the members of the colony spread 
themselves out towards the source of illumina- 
tion. In other words, they are irritable, 
which is the technical way of expressing the 
fact that they are capable of responding by 
a movement to a stimulus—in this instance, 
to the stimulus of light. 
Under suitable conditions of temperature 
B 
