54 CHLOROPHYCEAE 



walled akinetes that germinate to form aplanospores from which new 

 plants arise. In P. japonica sexual reproduction is brought about 

 by macro- (sixteen per cell) and microgametes (sixty-four per cell) 

 that are both produced on the same plant so that this species, at 

 least, is anisogamous. The shape of P. crispa has been shown to 

 vary considerably with the habitat, the optimum conditions being 

 those where there is an abundant supply of nitrogen, such as may 

 be found in areas occupied by bird colonies. The genus, which is 

 generally absent from the tropics and subtropics, is represented by 

 saline, fresh-water or subaerial species, the latter being tolerant 

 towards considerable desiccation and temperature changes. This 

 resistance is attributed to the lack of vacuoles in the cells and also 

 to the high viscosity of the protoplasm. Water supply appears to be 

 the principal factor limiting successful development, especially in 

 the subaerial species. Some authors consider that the genus is 

 characterized sufficiently to warrant removal from the Ulotrichales, 

 but such a change does not really seem to be justified. 



Sphaeropleaceae : Sphaeroplea {sphaero, sphere; plea, full). 

 Fig. 41. 



This genus is widely distributed, being most abundant on ground 

 that is periodically flooded by fresh w^ater. The long, free, un- | 

 branched filaments consist of elongated coenocytic cells containing 

 one to seventy annular parietal chloroplasts. These latter have ^ 

 denticulate margins and occupy the periphery of disks of cyto- 

 plasm, the disks being separated from each other by vacuoles, 

 although occasionally they may come together to form a diffuse 

 network. Each disk normally possesses one or two nuclei in its 

 cytoplasm. In most of the species the septa develop as ingrowths, 

 though in S. Africana these are replaced by a series of processes 

 which appear to be comparable to the strands of a Caulerpa (cf. p. 

 91), but as they sometimes fail to meet at the centre the coeno- 

 cytes may be continuous. 



Vegetative reproduction is secured by means of fragmentation 

 and there is apparently no asexual reproduction. In sexual repro- 

 duction although the cells do not change in shape, nevertheless both 

 oogonia and antheridia are formed singly or in series, the plants 

 being either monoecious or dioecious. In the formation of oogonia 

 the annular chloroplasts first become reticulate and then the ova 



