60 



PLANT MORPHOLOGY 



organized. These escape through a terminal pore and are fertilized in 

 the water. The zygote gives rise at once to a vegetative plant. The 

 vegetative plants of Codhim are diploid and the reduction of chromosomes 

 takes place in connection with the formation of the gametes. 



Bryopsis. Some of the marine Siphonales are highly branched, one of 

 these being Bryopsis. The thallus is composed of a prostrate rhizome- 

 like portion, anchored by rhizoids, and an upright feathery portion, the 



Fig. 49. A small portion of the vegetative body of Bryopsis, showing branches of limited 

 growth, X75. 



latter consisting of an axis with branches of limited growth (Fig. 49). 

 In the formation of a gametangium, a branch is cut off by a cross wall and 

 gives rise to numerous bicilate gametes. A gametangium produces 

 either male or female gametes and these are usually borne on different 

 plants. The female gametes are about three times as large as the male 

 ones. Both escape into the water, where they pair and fuse. The 

 zygote secretes a cell wall and germinates immediately to form a new 

 vegetative plant. There are no spores of any kind in the life cycle. The 

 reduction of chromosomes occurs when the gametes are formed, and thus 

 the vegetative plant is diploid. 



Caulerpa. This is a marine form of interest because of the high degree 

 of differentiation of its coenocytic plant body (Fig. 50) . It consists of a 



