6o CHLOROPHYCEAE 



and there are two theories that have been put forward to explain its 



origin : 



(a) The group arose independently from flagellate organisms 

 which possessed a ring of flagellae. If this is true then there could be 

 no real connexion with the other members of the Chlorophyceae. 



(b) Several divisions of the two original blepharoplasts and 

 flagellae took place, thus resulting in the ring structure. If 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. De- 

 velopment 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 persists as 

 the attachment organ and the upper cell goes on to form the new 

 filament. 



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

 protoplast grows out pushing it aside, and when the protoplast has 

 reached a certain length a cross-wall is formed at the junction 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 ^H and 

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

 antheridia are produced on the same plant {ynonoecioiis forms) : in 

 other species the oogonia and antheridia appear on different 

 filaments which are morphologically alike {dioecious hgmothallic 

 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 hetero- 



