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 algce 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 



