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 denned 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 is 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 



