ii8 



XANTHOPHYCEAE 



symbiotically with this alga and colour it yellow or brown from 

 ferric carbonate. This substance controls the pU of the water and 

 thus acts as a local buiTer for the alga whilst the bacteria obtain 

 their oxygen requirements from the Trihonema. 



BoTRYDiACEAE : Botrydiutti (a small cluster). Fig. 83. 



This genus belongs to the Heterosiphonales and is analogous to 

 a form such as Protosiphofi, the commonest species, Botrydium 

 gramilatiwiy being frequently confused with it, especially as these 

 two plants are often associated on areas of drying mud. B. granu- 

 latum makes its appearance during the warmer part of the year 

 when it is seen that the green, pear-shaped vesicles are rooted 

 by means of colourless dichotomously branched rhizoids. The 



GAMETES 



ZYGOTE 



Fig. 83. Botrydium granulatum. A, plant. B, swarmer. C, cyst formation. 

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



membrane is composed of cellulose and the lining cytoplasm con- 

 tains numerous nuclei scattered throughout it, whilst the chloro- 

 plasts, containing pyrenoid-like bodies, are confined to the aerial 

 part. The shape of 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 re- 

 production 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 submerged aplanospores are formed instead. 

 Under dry conditions each vesicle develops into a single cyst 



