104 



BOTANY 



B 



FIG. 74. Germinating zoospores 

 of (Edogonium sp. ; f, holdfast 

 (X550). 



ferva), or a crown of numerous cilia ((Edogonium). There is gen- 

 erally an eye-spot like that in the cells of the Volvocacese, and there 



may also be contractile vacuoles, so 

 that the resemblance to the simpler 

 Volvocaceae is very striking. After a 

 brief period of activity, during which 

 the cells show marked sensitiveness to 

 light, they come to rest, and secrete 

 a thin cellulose membrane. The cili- 

 ated end becomes attached, and soon 

 develops a disk or root-like organs 

 which anchor it to the substratum. 

 The cell elongates, and, dividing re- 

 peatedly, develops quickly into the 

 characteristic filament. 



In certain forms (e.g. Stigeoclonium) 

 the plants often remain for a long time 

 in a unicellular condition, the cells 

 separating when they divide, and thus 

 producing large colonies of unicellular 

 plants, which are scarcely distinguisha- 

 ble from many Protococcoideae. Sooner 

 or later, these cells develop into the filamentous form of the mature 

 Alga. 



Aplanospores. Less commonly the non-sexual reproduction is due 

 to the formation of Aplanospores ; i.e. the- cell-contents contract, and 

 develop a new cell-wall within the mother-cell, thus forming resting- 

 spores, which later develop into new plants. Very similar are the 

 so-called "Akinetes," which differ from the Aplanospores only in 

 having the cell-wall derived from the wall of the mother-cell. 



Sexual Reproduction. While sexual reproduction has not yet been 

 demonstrated for all the Confervoidese, it is probable that it always 

 occurs. This is seen in its simplest form in Ulva and Ulothrix 

 (Fig. 71, F). The gametes in these forms are quite similar (Ulva), 

 or there may sometimes be a slight difference in size. They are dis- 

 tinguished from the swarm-spores by their smaller size and by having 

 two cilia instead of four. The zygote resulting from their conjuga- 

 tion may either germinate at once (Ulva) or it may become a rest- 

 ing-spore, from which are developed swarm-spores. 



In all of the higher types, the differentiation of the sexual cells is well marked, 

 and the egg-cell is quite destitute of motion. The cells containing the gametes 

 are generally more or less modified. This is well seen in (Edogonium (Fig. 75), 

 where the oogonium, the cell containing the egg-cell, is a good deal enlarged, 

 while the antheridium, containing the male cells (spermatozoids), is much 

 smaller than the vegetative cells. In the peculiar genus Sphaeroplsea, whose cells 



