44 



PLANT MORPHOLOGY 



protoplasts of which escape into the water as biciHate sperms (Fig. 31C). 

 An oogonium is formed near the margin of the thallus by the enlargement 

 of a vegetative cell, its protoplast becoming a nonmotile egg. In the 

 branched species the antheridia and oogonia are borne at the ends of 

 separate branches. Here the oogonium has a long, slender extension 

 (trichogyne) with a terminal opening. A few species are dioecious. 



A sperm enters the oogonium and fertilizes the egg, the zygote enlarg- 

 ing and becoming thick-walled. At the same time adjacent vegetative 

 cells grow up around the oogonium and form a case (Figs. 3 ID and 32.4). 



Fig. 32. Coleochaete pulvinata. A, section of oogonium containing a zygote and sur- 

 rounded by jacket produced by adjacent vegetative cells; B, section of oogonium containing 

 a group of cells derived from the zygote, each of which gives rise to a zoospore. (After 

 Oltmanns.) 



After undergoing a period of rest, the zygote germinates inside the 

 oogonium and produces a spherical body consisting of 16 or 32 cells, 

 each cell in turn producing a biciliate zoospore (Fig. 32J5). This escapes 

 and gives rise to a new vegetative plant. In the discoid species the 

 zygote produces an eight-celled body. The reduction of chromosomes 

 takes place when the zygote germinates. Consequently, the body of 

 spore-producing cells that develops from it is haploid and so cannot be 

 regarded as a sporophyte. Thus Coleochaete is without a true alternation 

 of generations. 



Summary. The plant body of the Ulotrichales is multicellular (except 

 in Protococcus) , being either a simple filament, a branched filament, or a 

 flat plate-like thallus. The cells contain one nucleus and a single chloro- 

 plast. Growth occurs by division of the vegetative cells. Nearly all 

 members produce zoospores, these being either biciliate or quadriciliate. 

 Asexual reproduction may also occur by aplanospores or by akinetes. 

 Sexual reproduction ranges from isogamy to heterogamy. 



