414 



BOTANY 



TART II 



plants. The female gamete becomes attached to a substratum, and numerous 

 male gametes gather around it (Fig. 355, 1). Ultimately a male gamete fuses 

 with the female to form a zygote (Fig. 355, 2-9). This contains after the fusion 

 a single nucleus, but two chrornatophores, and soon becomes attached and 

 surrounded by a cell wall ; it grows into a new plant. 



In other Phaeophyceae the distinction between the two kinds of gametes is 

 expressed in their shape and size. The Cutleriaceae afford a particularly good 

 transition from isogamy to oogamy and a differentiation of the gametangia into 

 antheridia and oogonia (Fig. 356). 



In certain Phaeosporeae, e.g. the Cutleriaceae ( 43 ), a regular alternation of genera- 

 tions is found. The haploid sexual plants alternate with diploid asexual individuals, 



FIG. 357. Dictyota dichotoma. Transverse sections of the thallus. 1, With tetrasporangia ; 

 with a group of oogonia ; 3, with a group of antheridia (after THURET). A, Spermatozoid 

 (after WILLIAMS). (From OLTMANNS' Algae.) 



the reduction division taking place in the zoosporangia. In Zanardinia the two 

 generations are alike and have a disc-shaped thallus. In Cutleria, on the other 

 hand, they are unlike, the sexual plant having erect, dichotomously-divided shoots, 

 while the sporophytic plants (Aglaozonia) form flat, lobed, prostrate discs. The 

 alternation of generations is not always strictly maintained in Cutleria, since both 

 sporophyte and gametophyte may give rise to its like. 



Order 2. Tilopteridaceae ( 44 ) 



This order includes only a few forms which in vegetative, habit correspond 

 to the simpler Phaeosporeae such as Ectocarpus. A single large egg-cell is 

 produced in each oogonium, while the antheridia give rise to small biciliate 

 spermatozoids. In the asexual sporangia there is no division into spores, but the 

 contents become a single large monospore with four nuclei and enclosed by a cell 

 wall, while in the Dictyotaceae four free spores are formed. 



