CHLOROCOCCALES 41 



nucleate the mature ones may possess as many as eight nuclei. 

 Biflagellate zoospores are formed, the number depending upon the 

 external physical conditions, and they are usually liberated at day- 

 break from the parent cell into an external vesicle in which they 

 swarm for a time, but they soon become arranged into a new 

 coenobium before the vesicle ruptures. The flagellae are some- 

 times absent. Isogametes are also formed and liberated singly, and 

 after fusion the zygote divides up into a number of swarmers ; each 

 of these subsequently turns into a thick-walled polyhedral cell in 

 which a new coenobium is formed. There would seem to be very 

 little justification for placing this and the next genus into the 

 Siphonales, as some authors have suggested, because their mode of 

 reproduction is essentially much more akin to that of the Chloro- 

 coccales. 



*Hydrodictyaceae : Hydrodictyon (hydro, water; dictyon, net). 



Fig. 31- 



• 



The number of species are few, the commonest, H. reticulatum, 

 having a world-wide distribution though it occurs but rarely in 

 each locality. It is a hollow, free-floating, cylindrical network 

 closed at either end and up to 20 cm. in length. The individual 

 coenocytic cells are multinucleate and are arranged in hexagons or 

 pentagons to form the net. The chloroplast is reticulate with 

 numerous pyrenoids, though in the young uninucleate cells there 

 is but a simple parietal chloroplast which later becomes spiral and 

 then reticulate. H. africanum and H. patenaeforme develop into 

 saucer-shaped nets, the former with spherical cells up to i cm. 

 diameter which may become detached and lie on the substratum 

 looking like pearls. The other species is composed of cells which 

 may grow up to 4 cm. long by 2 mm. in diameter. Asexual 

 reproduction in H. reticulatum is by means of numerous uninucleate 

 zoospores which swarm in the parent cell about daybreak and then 

 come together to form a new coenobium which is subsequently 

 liberated, further growth being brought about by elongation of the 

 coenocytic cells. It is interesting to note that the arrangement of 

 the daughter cells in the parent coenocyte agrees with the me- 

 chanical laws for obtaining the greatest rigidity with the maximum 

 economy of space. 



Asexual reproduction is unknown in H. africanum and H.patenae- 



