GROUP I. THALLOPHYTA : ALG^E. 243 



differentiation into a female cell comparable with the oosphere of 

 the oogonium. 



The male organ, the anther idium, is with few exceptions (e.g. 

 Sphaeroplaea) more or less differentiated in form, attaining its 

 highest development in the Characeae. It is usually unicellular ; 

 but in (Edogonium it consists of two cells, and of many cells in 

 the Characese where its structure is highly complex. When the 

 antheridium is unicellular, it usually gives rise to a number of 

 male cells, but in Coleochaete and most Rhodophyceae only to one. 

 When it is multicellular, each fertile cell gives rise to a single 

 spermatozoid. 



Sexual organs are not known in the following forms : Cyano- 

 phyceae, some Protococcoideae, some Siphonaceae, some Phaeosporese 

 (e.g. Desmarestia ; Laminariaceae, except Chorda). 



The asexual reproductive cells of the Algae are formed either 

 sexually or asexually : the former are either zygospores, or oospores ; 

 the latter are spores. 



The zygospores and oospores occur exclusively in the Chloro- 

 phycese and Phaeophyceae : they are spherical nucleated cells with 

 a cell-wall. The differentiation of the cell-wall varies with the 

 nature of the spores and of the conditions to which they are 

 likely to be exposed. In the Phaeophyceae the zygospore or the 

 oospore germinates at once on its formation, and its wall remains 

 thin, and consists only of a single layer. In the Chlorophyeeae, the 

 sexually produced spore usually undergoes a period of quiescence 

 before it germinates, and except in marine forms (e.g. Acetabularia), 

 it is exposed during this period to the danger of desiccation. As a 

 protection, its wall consists of two layers, a delicate endospore, and 

 a hard cuticularised exospore which often grows out into promin- 

 ences, giving to the spore a stellate appearance. 



The spores produced asexually may be resting-spores with a 

 thick wall, which may consist of two layers as described above 

 (e.g. many Cyanophyceae) : or cells destitute of a cell-wall, either 

 ciliated (e.g. zoospores of (Edogonium, Coleochaete, Sphaeroplaea, 

 Pandorina), or not ciliated (e.g. tetraspores and carpospores of the 

 Rhodophyceae). 



The asexual reproductive organs are sporangia. In the simple 

 unicellular forms, the whole body may become a sporangium (e.g. 

 Hsematoccoccus) : in some coenobitic 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, 



