230 PART III. THE CLASSIFICATION OF PLANTS. 



Ectocarpus), or not ciliated (e.g. tetragon idia of the Rhodo- 

 phyceae). 



The asexual repoductive organs are termed gonidangia when borne 

 by the gametophyte, and sporangia when borne by the sporophyte. 

 In the simple unicellular forms, the whole body may become a 

 sporangium (e.g. Haematococcus) : in some ccenobitic multicellular 

 plants, there are no definite asexual reproductive organs, but any 

 of the cells of the body may act as such (e.g. Ulothrix, Pandorina, 

 Coleochaete, Ulva) without any special morphological differentia- 

 tion ; this is true likewise of the ccenocytic Algae (Siphonoideae, 

 such as Botrydium, Vancheria, Sphaeroplea, Cladophora) where the 

 whole or part of the body may act as a gonidangium. Specially 

 differentiated gonidangia occur only in some Phaeosporeae, and 

 in the Rhodophycese where they usually produce each four gonidia 

 (tetragonidia) and are hence termed tetragonidangia : specially 

 differentiated sporangia are developed in the sporophyte (cysto- 

 carp) of the Rhodophyceae where they are termed carposporangia, 

 and probably also in some Phaeosporeae : these organs are in all 

 cases unicellular. 



In the Cyanophyceae the formation of spores is effected without 

 any sporangium, for in these plants a cell of the body is converted 

 into a spore by simple encystment. 



As a rule a sporangium (or a gonidangium) gives rise to a num- 

 ber of spores (or gonidia) ; but only one gonidium is formed in 

 the gonidangium of Vaucheria (see p. 115) and of (Edogonium, 

 and one spore in the carposporangium of the Rhodophyceae. 



The Life-History of the Algce. No alternation of generations can, 

 of course, be traced in those Algae, already enumerated, in which 

 either asexual or sexual reproduction is unknown. In those in 

 which both these modes of reproduction occur on distinct forms, 

 there is usually an indication of alternation of generations, though 

 it has not been traced in all cases. Thus, in most Chlorophyceae, 

 there is an alternation of this sort (e.g. Ulothrix, Pandorina, 

 CEdogonium, Sphaeroplea), that the zygospore or oospore represents 

 the sporophyte, constituting in fact a unicellular sporangium 

 which produces two or four zoospores which, on germination, give 

 rise to gametophytes. In Coleochaete the oospore undergoes cell- 

 division, forming a small multicellular sporophyte each cell of 

 which eventually gives rise to a zoospore. 



In the Phaeophyceae, an alternation of generations appears to 

 occur in the Cutleriaceae, and possibly in some other forms. The 



