CHAPTEE XXI 



SUMMARY OF THE LIFE HISTORIES AND EVOLUTION OF 

 THE 



247. The life histories of the algae. The life history of a 

 plant is the succession of stages leading from one generation to 

 another and of course includes the reproductive periods. Bepro- 

 duction may be as simple a process as the breaking off of 

 portions from the parent plant, called vegetative reproduction. 

 Almost all groups of plants have developed some forms of 

 vegetative reproduction. The detached portions may be merely 

 fragments, as in Oscillatoria (Sec. 209), or much more compli- 

 cated, as certain bud-like structures in some of the brown and 

 red algae. Methods of vegetative reproduction give very simple 

 life histories, which are merely a succession of similar forms 

 such as may be represented by the formula 



P-P-P-P, etc., 

 P standing for the plant type. 



The commonest methods of reproduction in the algse are 

 through the special cells called spores, which may be asexual 

 in character or formed sexually. The commonest form of spore 

 reproduction in the algae is through the zoospore. When there 

 is no sexual process in the life history, but some method of 

 asexual spore reproduction, the formula of the life history 

 becomes p _ aex ^ S ._ P _ asex . s . _ P> e tc, 



asex. s. standing for asexual spore. 



The development of sex in a plant complicates at once the 

 life history. Gametes are formed, which unite to give sexually 

 formed cells or spores. These spores may develop directly into 

 plants like the parents, as in Sphcerella and Volvox, Spirogyra, 



221 



