THE EVOLUTION OF THE ALG^E 225 



f 



of protecting the egg. It will appear later that the eggs of liver- 

 worts, mosses, and ferns are retained in special, protective, cellular 

 structures called arehegonia, which are the female reproduc- 

 tive organs. The presence of this organ is one of the important 

 characters of these groups (bryophytes and pteridophytes), and 

 its absence is one of the peculiarities of the thallophytes. 



249. The evolution of the algae. The evolution of the algte 

 is the result of many factors which affect their life habits and 

 life histories. Sexual processes have been the chief factors 

 modifying life histories, for they are always a stimulus to devel- 

 opment, and have been the starting points for some of the most 

 important complications in the life histories and developments 

 of groups. The most conspicuous illustration of this principle 

 appears in the red alga?, where an asexual generation follows the 

 sexual process, and similar conditions are present in a peculiar 

 group of the brown algre represented by Dictyota. 



One of the most clearly marked evolutionary principles illus- 

 trated in the algae is the tendency to establish fixed or attached 

 forms and to limit the motile stages in the life history to the re- 

 productive cells (zoospores or gametes). Some of the lower algas 

 are motile throughout almost all of their life histories, as in the 

 Volvocacece (Sec. 215) and that group of uncertain relationships, 

 the flagellates (Sec, 204). But the motile stages become merely 

 reproductive phases in the higher forms. Thus the appearance 

 of zoospores and motile gametes in the life histories of higher 

 types of alga? is believed to represent a return for a short time 

 to the motile conditions and habits of their ancestors. 



The establishment of attached plant bodies opened immense 

 possibilities of plant development, and resulted at once in a 

 great variety of structures. The first of these were simple forms 

 of thalli, such as filaments, membranes, and plates of cells. But 

 later the plant structures became more complex, developing 

 holdfasts and stems, which bore leaf-like lateral structures, 

 evidently differentiated to give a large exposure to sunlight, 

 and for the work of photosynthesis. Thus types of plant bodies 



