THE PLANT AND ITS FOOD 19 



of movements which make for locomotion. 

 Such movements indeed soon come to lose 

 their value, even in water plants, when the 

 capacity for ingesting solid food has been lost, 

 whilst they would tend to render the exist- 

 ence of a land flora practically impossible. 



We may consider Chlamydomonas, then, as 

 a plant belonging to a class the members of 

 which have not as yet diverged far enough 

 along the plant line of evolution to have 

 lost the power of movement. But even 

 amongst the near relatives of the species under 

 consideration there are forms which pass at 

 least a part of their vegetative lives in the 

 passive and non-motile condition character- 

 istic of more advanced members of the 

 vegetable kingdom. A familiar example is 

 afforded by the green incrustation everywhere 

 to be seen on old damp palings. This in- 

 crustation consists of countless numbers of 

 minute green cells known as Pleurococcus, 

 which grow and multiply by division. The 

 separate individuals are habitually destitute 

 of all locomotory mechanism, and each grows 

 and multiplies in the spot where it happens 

 to have become fixed. 



The Pleurococcus plant thrives in damp air, 

 and it depends on the chance supplies of 

 moisture for the water it requires. The gases 

 of the atmosphere, passing by diffusion 

 through the membrane or cell wall, are dis- 

 solved in the watery sap which bathes its 

 living protoplasmic substance. Thus supplied 



