GROUP I. THALLOPHYTA : ALGS. 229 



differentiation into a female cell comparable with the oosphere 

 of the oogonium. 



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

 Sphaeroplaea) more or less differentiated in form, attaining its 

 highest development in the Characeas. It is usually unicellular ; 

 but in (Edogonium it consists of two cells, and of many 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 Protococcoidese, some Siphonaceae, Syngeneticae, some 

 Phaeosporeae (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, those produced by the gametophyte (when 

 this is the case) being distinguished as gonidia. 



The zygospores and oospores occur exclusively in the Chloro- 

 phyceae 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 Chlorophyceaa, the 

 sexually produced spore (except in Botrydium under favourable 

 conditions) undergoes a period of quiescence before it germinates, 

 and, except in marine forms (e.g. Acetabularia, Dasycladus), it is 

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

 protection, its wall consists of two layers (see p. 69), a delicate 

 endospore, and a hard cuticularised exospore, which often grows 

 out into prominences, giving to the spore a stellate appearance. 



The spores produced asexually by the sporophyte may be rest- 

 ing-spores with a thick wall, which may consist of two layers 

 as described above (e.g. many Cyanophyceas), or cells destitute of 

 a cell-wall, either ciliated (e.g. zoospores of (Edogonium, Coleo- 

 chaete, Sphaeroplaea, Pandorina), or not ciliated (e.g. carpospores 

 of the Rhodophyceae). 



The gonidia produced by the gametophyte are never " resting- 

 spores," but are destitute of a cell- wall, and are either ciliated 

 (e.g. zoogonidia of Vaucheria, (Edogonium, Coleochaete, Ulothrix, 



