EUPHYCOPHYTA 



97 



in diameter, arising from a slender branched tuberous rhizoid 

 embedded in calcareous Lithothamnion (cf. p. 238) growing at or 

 below low-tide mark. There are only a few species of Halicystis^ 

 one at least of which possesses pyrenoids, though all contain 

 numerous nuclei in the peripheral cytoplasm. There does not ap- 

 pear to be any cellulose in the material composing the cell wall. 

 Swarmers develope in the cytoplasm at the apex of the vesicle in an 



Fig. 54 Halicystis ovalis (and Derbesia marina). A, plant of 

 Halicystis liberating gametes. B, rooting portion oi Halicystis show- 

 ing old rhizome and Hne of abscission (a) and new vesicle (6). C, 

 gathering of protoplasm to form gametes. /=Uning cytoplasm, 

 p =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, 

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



area which becomes cut off by a thin cytoplasmic membrane, the 

 area thus cut off representing a gametangium. Macro- and micro- 

 gametes are formed and forcibly discharged in the early hours of the 

 morning through one or more pores. There are several crops of 

 these swarmers produced by successive migrations of cytoplasm 

 into the apical areas at bi-weekly intervals coincident with the 

 spring tidal cycles. Fertilization occurs in the water, and the zygote 

 in H. ovalis germinates into a branched protonemal thread that in 

 three months has developed into a typical Derbesia plant with the 

 erect aerial filaments arising from the basal rhizoidal portion. 



It has been demonstrated only quite recently that both Halicystis 

 ovalis and Derbesia marina are simply two stages in the Ufe cycle of 

 one alga, but in addition to the evidence from cultures the two 

 species have the same geographical distribution. The mature Der- 



