80 THE ALGAE 



plants these may be broken off. These bristles develop above a pore 

 in the cell wall through which the protoplast extrudes, whilst at the 

 same time a membrane is secreted over the protruding bare proto- 

 plast. Asexual reproduction takes place in spring and early summer 

 by means of biflagellate zoospores which have no eye-spot and are 

 produced singly. After a motile phase the zoospore settles down 

 and divides either (a) horizontally, when the upper segment 

 develops into a hair and the lower forms the embryo disc, or (b) 

 vertically, when each segment grows out laterally; in either case it 

 will be noted that hair formation takes place at a very early stage. 



Sexual reproduction is by means of a speciahzed oogamy, some 

 of the species being dioecious and the remainder monoecious. The 

 female organs, or carpogonia, are borne on short lateral branches 

 and subsequently undergo displacement. Each carpogonium pos- 

 sesses a short neck or trichogyne (the long neck of Coleochaete 

 scutata being an exception) the top of which bursts when the car- 

 pogonium is mature. In the disc forms the carpogonia originate as 

 terminal bodies on the outside of the disc, but as the neighbouring 

 cells continue growth they eventually become surrounded and 

 appear to be in the older part of the thallus. The antheridia develop 

 in clusters at the end of branches (C. pulvinatd) or from prostrate 

 cells. They finally appear as small outgrowths cut off from a mother 

 cell with stages in their development that are strongly reminiscent 

 of the Rhodophyceae (cf. p. 217). Each antheridium produces one 

 biflagellate colourless antherozoid which has been contrasted with 

 the non-motile rhodophycean spermatium. 



After fertilization the neck of the carpogonium is cut off and the 

 basal part enlarges; branches arise from the underlying cells and 

 eventually surround the oospore where they form a red or reddish 

 brown wall, though in the disc forms this wall is only formed on the 

 side away from the substrate. At the same time the enclosed oospore 

 develops a thick brown wall and the cells of the outer envelope then 

 die. The oospore, or spermocarp, hibernates until spring when it 

 becomes green and divides into sixteen or thirty-two cells, and 

 these, when the wall bursts, each give rise to a single swarmer 

 which must be regarded as a zoospore. Meiosis takes place at the 

 segmentation of the zygote so that there is only the haploid genera- 

 tion. On the other hand, some observers have recorded the develop- 

 ment of dwarf asexual plants before the reappearance of new sexual 

 ones, but this is a phase of the life history that demands re-investi- 



