6o 



THE ALGAE 



duce large, thick-walled akinetes that germinate to form aplano- 

 spores from which new plants arise. In P. japonica, sexual repro- 

 duction is brought about by non-motile macro- (i6 per cell) and 

 motile micro-gametes (64 or 128 per cell) that are both produced 

 from the same plant so that this species at least is monoecious 

 and anisogamous. Despite the existence of sexual reproduction 

 multiplication in this species is usually by asexual means. The 

 plants are haploid (n=3)5 but in view of recent discoveries about 

 P. stipitata it is desirable that P.japonica be restudied. 



Fig. 31 Life cycle of Prasida stipitata. Meiosis (RD) occurs in 

 peripheral cells of diploid thallus. (Modified from Friedmann.) 



In P. stipitata the adult plant is diploid (n = 12-14) and repro- 

 duces by aplanospores. Subsequently cells, termed meiospores, at 

 the distal periphery undergo reduction division and give rise to 

 leafy appendages, the cells of which are haploid. These appendages 

 are monosexual and give rise to either male or female gametes that 

 are anisogamous. The gametophyte plants are therefore attached 

 to the diploid plant. It has been observed that wholly diploid plants 

 occupy the upper levels of a P. stipitata zone and plants with game- 

 tophytes occur lower down. 



The shape of the thallus in P. crispa varies considerably with the 

 environment, the optimum conditions being where there is abun- 

 dant nitrogen, such as may be found in areas occupied by bird 

 colonies. The genus, which is generally absent from the tropics 

 and sub-tropics, is represented by saline, fresh-water or terrestrial 

 species, the last-named being tolerant towards considerable desic- 

 cation. This is attributable to the lack of vacuoles in the cells and 



