280 



ScMzogoniales 



Vegetative propagation often occurs by a proliferation (or gemmation) of 

 the thallus, resulting in the detachment of small portions from the margin. 



Asexual reproduction occurs by akinetes, formed either directly from the 

 vegetative cells or by the division of the vegetative cells in two planes to 

 form ' tetraspores ' (Lagerheim, '92 B). The akinetes may grow directly into 

 new plants on liberation or they may become aplanosporangia in which a 

 number of rounded or ovoid aplanospores are formed. The latter escape by 

 the bursting of the wall of the aplanosporangium and grow directly into new 

 plants (Wille, '02 ; '06). Under unfavourable conditions both akinetes and 

 aplanospores may become resting-spores. 



Zoogonidia and gametes are entirely unknown. 



m 



i 



IH 



p 

 in 





Fig. 179. A C, Prasiola crispa forma muralis ( = Schizogonium murale Kiitz.), x 500. D G, 

 Prasiola crispa (Lightf.) Menegh. ; D, two expanded thalli, nat. size; E, simple filament, 

 x 500 ; F, part of irregular filament, x500; G, basal part of an expanded thallus showing 

 a few rhizoids, x 400. 



The various species of Prasiola are subaerial Algae, living on damp ground, damp 

 walls, and on rocks, especially rocks of the sea-shore. In some forms the Hormidium- 

 and 8chizogonium-&\&ie& appear to be permanently retained, whereas others almost 

 invariably form expanded thalli. Prasiola crispa forma muralis ( = Schizogonium murale} 

 is invariably filamentous and is found almost exclusively on damp earth, especially in 

 towns and villages, where it often forms a green stratum between paving stones (fig. 179 

 A (7). P. crispa (fig. 179 D G} has an expanded thallus occurring on damp earth under 

 walls, and also in quantity on rocky sea-shores and about the nesting-places of sea-birds. 

 It has a world-wide distribution, extending far into the Arctic and Antarctic regions, 

 always occurring where there is a plentiful nitrogenous food-supply. In one form of this 

 Alga, which has been described under the name of Gayella polyrhiza Eosenvinge, a cell- 

 mass is formed instead of a cell-plate. 



Gay ('91), Chodat ('02) and Collins ('09) separate the genera Prasiola and Schizogonium, 

 but other authors (notably Wille, '00; '09s; Brand, '14) have united them under Prasiola. 



Wille, and also Borgesen ('02), have each described some interesting marine forms of 

 Prasiola in which the plants are more amply supplied with rhizoids. The expanded 

 thallus in these forms does not attain so large a size as in the terrestrial forms. The 

 thallus of P. crispa is normally from 4 mm. to 2 cms. in length, but in the vicinity of 

 sewage works and on ' contact beds ' the thalli may attain a length of 8 cms. 



