48 THE ALGAE 



Botrydium, in the damp mud at the edges of ponds. It contains 

 starch whereas Botrydium (see p. 264) does not. The green aerial 

 portion is more or less spherical, up to loo/x in diameter, grading 

 into a colourless rhizoidal portion that may branch occasionally. 

 The plastid, which contains numerous nuclei and pyrenoids, is 

 parietal and reticulate. The shape and colour of the thallus can be 

 modified by varying the external conditions; bright Hght and low 

 moisture for example causes an old thallus to turn brick red. During 

 dry weather the contents of the vesicle encyst to give large aplano- 



ores 



K% 



Fig. 22 

 C, cyst 



^ Partition 



Protosiphon botryoides. A, B, plants, one showing budding, 

 formation. D, zygote ( x 1666). E, schematic diagram of life 

 cycle. (A-C, after Fritsch; D, E, after Bold.) 



Spores or *coenocysts'. When conditions are once more favourable 

 these cysts either grow directly into vegetative cells or else produce 

 biflagellate zoospores. 



Vegetative reproduction can take place by lateral buds which 

 become cut off by cell walls. Flooding of plants on damp soil 

 causes them to produce biflagellate swarmers which behave as iso- 

 gametes though they can develop parthenogenetically. The zygote 

 either germinates directly or may remain dormant for some time. 

 The plant is probably haploid as there is evidence of meiosis at 

 zygote germination. Morphologically it is of great interest as a 

 possible source of origin for the Siphonales (see p. 310). 



