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 
