1 88 Plant Life and Evolution 



of the unicellular Volvocales may be taken to repre- 

 sent the commencement of the series leading up to 

 the higher plants. The earliest plants may be as- 

 sumed to have been motile like the existing Volvo- 

 cales, but the power of motion was probably early 

 lost in the vegetative cells, this being associated with 

 the power of photosynthesis, which does not make 

 it necessary for the plant to move about for food. 

 So long as the plant is completely immersed no 

 special organs are necessary for absorbing the water 

 with the dissolved food constituents, these being 

 taken in freely at all points of the surface. More- 

 over, such submerged plants are not subject to loss 

 of water through evaporation, and therefore the 

 superficial cells do not need to develop a cuticle. 



MARINE PLANTS 



The relation of the seaweeds to the simpler and 

 probably more ancient fresh-water algse is largely 

 a matter of speculation. The true brown algae 

 are hardly at all represented in fresh water, but 

 there are a number of fresh-water organisms which 

 may be remotely related to them. The development 

 of large amounts of gelatinous or mucilaginous tis- 

 sues which hold the water with great tenacity, and 

 also absorb water very rapidly when the plants are 

 wet by the rising tide, has already been referred to. 

 Some of the large kelps may be exposed to the 

 air for days before all of the moisture is lost from 



