EUPHYCOPHYTA 



59 



Prasiolaceae: Prasiola (prasio, green). Figs. 30, 31 



The young unbranched filament, which is known as the 'hor- 

 midium' stage, consists of a single row of cyUndrical cells with thick 

 walls which frequently exhibit striations. Later on the cells divide 

 longitudinally and produce a thin expanded thallus, known as the 

 'schizogonium' stage which tapers to the base. The cells of the 





Fig. 30. Prasiola. A, plant of P. crispa. B, 'Schizogonium' stage of 

 P. crispa forma muralis. C, D, 'Hormidium' stage of P. crispa f. 

 muralis with akinetes. E, development of macrogametes in P. 

 japonica ( x 665). F, development of microgametes in P. japonica 

 ( X 665). G, P. crispa^ membrane striations in ^Schizogonium' stage 

 ( X 650). H (a-d)i formation of aplanospores in akinetes and young 

 plants. (A, B, after Fritsch; C, D, H, after Oltmanns; E-G, after 



Knebel.) 



mature expanded thallus are often arranged in fours and possess 

 axile, stellate chloroplasts, whilst in some species another feature 

 is the presence of short rhizoids that occur in the stalk-like portion 

 or else are produced from the marginal cells. In the juvenile fila- 

 ment reproduction takes place by means of fragmentation as a 

 result of the death of isolated cells, whilst in the older, more leafy 

 thallus, *buds' can arise from the margin. Sometimes the cells pro- 



