68 THE ALGAE 



this is correct then development might well have occurred 



from a Ulotrichalean type of swarmer. 

 When the zoospore is ripe the cell wall ruptures near the upper 

 end and the swarmer is liberated into a delicate mucilaginous vesicle, 

 but this soon disappears, thus allowing the zoospore to escape. 

 After remaining motile for about an hour the anterior end becomes 

 attached to some substrate and develops into a holdfast, or else the 

 zoospore flattens to form an almost hemispherical basal cell. The 

 type of holdfast depends on the species and the nature of the sub- 

 strate, a smooth surface inducing a simple holdfast and a rough 

 surface inducing the development of a branched holdfast. Develop- 

 ment of the one-celled germling can proceed along one of two 

 lines, depending on the species : 



(a) The single cell divides near the apex by the normal method 

 described above, in which case the basal daughter cell per- 

 sists as the attachment organ and the upper cell goes on to 

 form the new filament. 



(b) The apex of the cell develops a cap and then a cyUnder of 

 protoplast grows out pushing it aside, and when the proto- 

 plast has reached a certain length a cross-wall is formed at 

 the jimction of the cylinder and the basal cell. The upper cell 

 subsequently develops along the normal lines. 



Sexual reproduction is by means of an advanced type of oogamy, 

 the development of sex organs being assisted by an alkaline pH and 

 some nitrogen deficiency. In some of the species the oogonia and 

 antheridia are produced on the same plant (monoecious forms) : in 

 other species the oogonia and antheridia appear on different fila- 

 ments which are morphologically ahke (dioecious homothallic forms). 

 The species belonging to both these groups are termed macrandrous 

 because the male filament is normal in size. There is a third group 

 of species in which the male filament is much reduced and forms 

 dwarf male plants. Such species are dioecious and heterothallic and 

 they form the nannandrous group. The dwarf males arise from 

 motile androspores which are formed singly in flat discoid cells, the 

 androsporangia, produced by repeated divisions of ordinary vegeta- 

 tive cells. The androspores may be formed either in the oogonial 

 filament — gynandrosporous species — or on other filaments that 

 do not bear oogonia — idioandrosporous species (Fig. 36). In shape 

 and structure the androspores are small editions of the zoospores, 

 and after swimming about they settle on the wall of the oogoniimi 



