2 BIOLOGICAL LECTURES. 



blances on the one hand to low animals, and on the other are 

 very like the free-swimming reproductive cells, or zoospores, 

 of many of the higher algae. The latter probably originated 

 from forms like the simpler Volvocineae by the loss of motil- 

 ity associated with the development of a continuous cellulose 

 membrane about the vegetative cells. This stage in the 

 evolution of the green algae is represented by some of the 

 simpler Protococcaceae. Above these unicellular forms are a 

 number of filamentous plants, single rows of nearly uniform 

 cells. Such plants are CEdogonium or Conferva, which well 

 represent the next step in the evolution of the plant body. 



The differentiation of the reproductive cells in the algae, 

 while it offers one of the most interesting and instructive 

 examples of the evolution of plant structures, must be passed 

 over here. It may be mentioned, however, that the differentia- 

 tion of the sexual cells has evidently taken place quite inde- 

 pendently in several groups of algae. 



In the further development of plant types two very important 

 factors are to be considered. First, the adaptation to a marine 

 existence, and, second, the exchange of an aquatic for a terres- 

 trial life. 



The conditions in the ocean are markedly different from 

 those in fresh water, and most plants which have adapted 

 themselves to life in the sea have become decidedly changed. 

 The salinity of the water has no doubt been one of the factors 

 in these changes, but more important is probably the question 

 of light. The two most characteristic groups of sea plants, 

 the red and brown seaweeds, are provided with special pig- 

 ments in addition to the chlorophyll, and there is little ques- 

 tion that these pigments are developed, in part at least, in 

 response to changed light conditions. 



As the conditions of light and temperature in a marine 

 environment are far more constant than those in fresh water, 

 we find, as a rule, that marine algae, especially those inhabiting 

 the deeper water, are more susceptible to changes of light and 

 temperature than are most fresh-water forms. 



While certain seaweeds growing between tide-marks are 

 subject- to exposure to the air, it is only for a short time, and 



