Factors in Evolution 31 



scmetimes a single cell may separate and give rise 

 immediately to a new filament. Many of these 

 lower filamentous algae may under certain condi- 

 tions assume a unicellular condition and this may 

 be induced artificially, this unicellular stage being 

 very marked when the plants are grown in concen- 

 trated culture solutions of high osmotic pressure. 



Somewhat higher in the scale are forms that 

 show polarity, this polarity being already deter- 

 mined in the free swimming spore from which the 

 plant arises. The forward end of the spore at- 

 taches itself, and probably in response to the contact 

 stimulus, develops a root-like organ. There is thus 

 a certain degree of specialization shown in the dif- 

 ferent parts of the plant and this becomes more 

 pronounced in the more highly specialized algae. 

 While in the simplest forms the cells are nearly 

 alike, in the larger and more complex types there 

 is a greater or less degree of specialization of the 

 tissues and organs more or less directly correlated 

 with the responses to various stimuli. For exam- 

 ple, organs resembling the leaves of the higher 

 plants occur in some algse, these leaf-like structures 

 being evidently organs especially adapted to the 

 work of photosynthesis. There also may be modi- 

 fications in these large algse of the tissues asso- 

 ciated with protection against loss of water, for 

 conduction, storage, etc., or adaptations enabling the 

 plant to withstand the beating of the surf. 



From these humble beginnings have gradually de- 



