GAMETES 



ZYGOTE 



Fig. 153 Botrydium granulatum. A, plant. B, swarmer. C, cyst 

 formation. D, diagram of life cycle. (A-C, after Fritsch; D, after 



Miller.) 



the vesicle is influenced by the environment, the shade forms being 

 elongate or club-shaped. In B. wallrothii the unbranched vesicle is 

 covered with lime whilst in B. divisum it is branched but without 

 lime. When the plants are submerged, reproduction takes place by 

 means of numerous zoospores which are set free by gelatinization 

 of the vesicle apex, but when the plants are only wet but not sub- 

 merged, aplanospores are formed instead. Under dry conditions 

 each vesicle develops into a single cyst (macrocyst) or into several 

 multinucleate spores (sporocysts), or else the contents migrate to 

 the rhizoids and there form several cysts (rhizocysts) which, when 

 conditions are again favourable, either germinate directly to a new 

 plant or else give rise to zoospores. In B. granulatum it is estimated 

 that about 40,000 isogametes are formed in each vesicle, but as the 

 plant is monoecious many fuse either in pairs or threes, rarely fours, 

 before they are Hberated. Those that do not fuse develop partheno- 

 genetically, although the stage at which meiosis occurs is not yet 

 known. The Hfe cycle can be tentatively represented as in Fig. 153. 



Vaucheriaceae: Vaucheria (after J. P. Vaucher). Figs. 154, 155 



This genus differs in many of its characters from other members of 

 the Siphonales, in which order it has been placed in the past. 

 Whereas many genera of the Siphonales are tropical, Vaucheria is 

 essentially temperate, inhabiting well-aerated streams, soil or saline 

 mud flats, and although some of the species (e.g. V. de haryana) 

 may be Ume-encrusted it is never to quite the same extent as in 

 some members of the Siphonales. The cell walls do not contain 

 s 265 



