266 



Siphonocladiales 



Asexual reproduction also takes place by means of thick-walled coenocysts, 

 formed from single segments which become free. These either germinate 

 directly or produce numerous zoogonidia after a period of rest. Gametes also 

 occur in both genera ; in Chsetomorpha they are green and isogamous, but in 

 Urospora they are anisogamous, the male gametes being smaller than the 

 female and almost colourless. The zygote germinates only after a period of 

 rest. Zoogonidangia and gametangia may occur simultaneously in the same 

 filament (Bb'rgesen, '02). 



Fig. 170. 1 and 2, Chsetomorpha aerea (Dillw.) Kiitz. 1, basal part of young plant ; 2, base of 

 an older plant. 3, cell of Urospora showing reticulate chloroplast. 4, part of a filament of" 

 Ch&tomorpha aerea during the formation of zoogouidia. (After Eosenvinge and Thuret, 

 from Oltnianns.) 



The genera are : Chsetomorpha Kiitzing, 1845, and Urospora 1 Areschoug, 1866. Both 

 genera are principally marine, but Ch. Linum (0. F. Mull.) Kiitz. often occurs in fresh 

 water. Most of the species are fixed to rocks on either exposed or sheltered coasts, 

 although some (Chsetomorpha brachygona Harv.) occur loose on a sandy or muddy bottom. 

 Species of both genera may also be epiphytic on larger Algse. 



1 From the remarks of Hazen ('02) it would appear that this genus should rightly be 

 regarded as a synonym of Hormiscia Fries, 1835. 



