422 



A TEXTBOOK OF BOTANY 



[Ch. X 



While in Ulothrix the filament is short and unbranched, 

 in Cladophora it is long and profusely branched, whereby are 

 formed those long, waving, bright green, hair-like masses 

 which, attached by one end to the bottom, wave back and 

 forth in slow-moving streams (Fig. 290). In the Sea Lettuce 

 (Fig. 291), an Alga common near low- tide mark on our sea 

 coasts, the plant body is a filmy, crinkled, bright green thallus. 



Conferva, another slender form of 

 fresh water, has zoospores with 

 cilia of unequal length, and is now 

 placed, with other forms display- 

 ing the same peculiarity, in a 

 separate class called Heterocontce. 

 Some of this group are epiphytic 

 land plants in the tropics. 



A marked advance in the mech- 

 anism of reproduction occurs 

 in (Edogonium, a form much like 

 Ulothrix in aspect and habit. 

 Here the zoospore, which is pro- 

 vided with a crown of cilia, is 

 very large (Fig. 292), for it is 

 composed of the undivided pro- 

 toplasm of a cell. In sexual re- 

 production, certain cells of the 

 filament enlarge and become 

 each an oogonium, the contents 

 forming a single huge egg cell. 

 The sperm cells, formed in smaller 

 cells of a filament, resemble the zoospores in miniature. In 

 fertilization, one of them enters the oogonium through a special 

 opening, and unites with the egg cell, the accompanying phe- 

 nomena being identical with those of fertilization in the higher 

 plants ; and thus is produced a thick-walled resting spore which 

 ultimately germinates to zoospores, as in case of Ulothrix. 

 In some species both kinds of sex cells arise on the same 



Fig. 291. — Ulva Lactuca (latis- 

 sima), the Sea Lettuce ; X !• 



The frond is colored bright 

 green, and is only two cell layers 

 thick. (After Thuret from Olt- 

 manns.) 



