296 



Ulotrichales 



but in these genera the main part of the thallus consists of basal recumbent 

 branches. 



Zoogonidia are formed either singly or rarely in twos, fours, or up to 32 



in a mother-cell, and they may arise from 

 almost any cell of a branch. The larger zoo- 

 gonidia are quadriciliated, with a prominent 

 pigment-spot and two contractile vacuoles, the 

 latter contracting alternately about every fifteen 

 seconds in Draparnaldia (Johnson, '93). Such 

 a zoogonidium arises singly in the mother-cell 

 and the first sign of its production is the 

 appearance of the pigment-spot, which is formed 

 at least twenty- four hours before the escape of 

 the mature swarm-cell. The orifice in the wall 

 of the zoogonidangium is often of much smaller 

 size than the zoogonidium, which assumes 

 various extraordinary shapes as it is squeezed 

 through. The colourless end of the zoogoni- 

 dium always comes through first and when free 

 the swarm-spore darts away with great rapidity. 

 Even when a number of zoogonidia are formed 

 in the mother-cell they do not on their escape 

 issue into a vesicle, but are expelled by the 

 swelling of a gelatinous substance which dis- 

 appears very rapidly in the surrounding water 

 (Johnson, '93 ; Pascher, '06 B). The smaller 

 zoogonidia are biciliated in certain genera and 

 there is every probability that in some cases 

 they are facultative gametes. 



The gametes are always biciliated and the 

 zygote rests for a short time before germination. 

 The zoogonidia as a rule only ' swarm ' for 

 about ten minutes, and they germinate at once 

 on coming to rest. The cilia are lost and the 

 cell rapidly increases in length, one pole generally 



elongating to form a hair-like multicellular outgrowth. The young plant 

 very soon begins to branch and in some genera a basal recumbent portion 

 is first developed, whereas in others the attachment is by a specially modified 

 basal cell, as in Thamniochtete aculeata (vide G. S. W., '04) and in some forms 

 of Stigeoclonium (Fritsch, '03) 1 . A variable development of rhizoids occurs 



1 Fritsch records the occurrence of a brownish-red salt of iron deposited about the 

 attachment-surface of the basal cell in St. variabile, which probably serves the purpose of 



Fig. 190. Draparnaldia platy- 

 zonata Hazen. A, small portion 

 of plant, x 70. B, single cell of 

 main filament showing the re- 

 ticulated chloroplast, x 180. 

 (From a specimen collected in 

 British Columbia by Mr F. L. 

 McKeever. ) 



