SIPHONALES 85 



Protosiphonaceae : Protosiphon {proto, first ; siphon, tube). Fig. 58. 



The single species common in north Europe grows in damp mud 

 at the edges of ponds, but a variety is also known from the desert 

 silt of Egypt which will tolerate temperatures up to 91° C. and salt 

 concentrations of at least i %. The green aerial portion is more or 

 less spherical, up to loo^u,. in diameter, grading into a colourless 

 rhizoidal portion that is occasionally branched. The chloroplast, 

 which contains numerous pyrenoids and nuclei, is an anomaly in 



Fig. 58. Protosiphon hotryoides. A, B, plants, one showing budding. c = chloro- 

 plast. C, swarmer formation. ^• = vacuole. D, cyst formation. E, zygote ( x 1666). 

 F, germination of zygote to form zoospores ( x 1026). G, group of plants grown 

 in a nutrient solution. (A-D, G, after Fritsch; E, F, after Bold.) 



the group because of its reticulate character. In very dry places the 

 rhizoid may be abbreviated to such an extent that the plant looks 

 like a Chlorococcum, whilst in cultures where nutrient conditions 

 are favourable one may obtain branched thread-like growths. The 

 shape of the thallus is determined by the incidence of the light, 

 unilateral light producing asymmetrical aerial portions, whilst 

 exposure to bright light and low moisture may also cause an old 

 thallus to turn brick red. During times of drought resting spores or 

 cysts are formed w^hich, when conditions become favourable once 

 more, either germinate directly or else produce zoospores, germina- 



