86 



CHLOROPHYCEAE 



tion in the desert forms occurring at temperatures between 12° and 

 35° C. Vegetative reproduction takes place by means of lateral 

 budding, but when submerged the plants also produce naked 

 biflagellate swarmers which usually act as isogametes, though they 

 are also capable of parthenogenetic development. Protosiphon 

 hotryoides is monoecious whilst the desert variety is dioecious, and 

 this fact alone would seem sufficient justification for regarding the 

 latter as a distinct species. The zygote either germinates immediately 

 to give a new plant or else may remain dormant for some time. 

 The plant is probably haploid, and morphologically is of great 

 interest in indicating how the more advanced Siphonales may have 

 arisen. 



*Halicystaceae : Halicystis {hali, salt ; cystis, bladder) and Derhesia 

 (after A. Derbes). Fig. 59. 



The gametophytic plants consist of an oval vesicle, up to 3 cm. 

 in diameter, arising from a slender branched tuberous rhizoid 



h 



Fig. 59. Halicystis oralis (and Derbesia marina). A, plant of Halicystis liberating 

 gametes. B, rooting portion of Halicystis showing old rhizome and line of 

 abscission (a) and new vesicle (6). C, gathering of protoplasm to form gametes. 

 / = lining cytoplasm, ^ = pore of dehiscence. D, male gamete ( x 600). E, female 

 gamete ( x 600). F, protonemal germling of Halicystis. G, Derbesia plant. H, 

 Derbesia, with zoosporangia, growing on Cladophora. I, Derbesia, zoosporangium. 

 J, Derbesia, zoospore. (A-C, F-J, after Fritsch; D, E, after Kuckuck.) 



embedded in calcareous Lithothamnia (cf. p. 317) growing at or 

 below low-tide mark. There are only two species of Halicystis^ one 

 of which possesses pyrenoids whilst the other does not, though both 

 contain numerous nuclei in the peripheral cytoplasm. There does 



