ULOTRICHALES 53 



*Prasiolaceae : Prasiola {prasio, green). Fig. 40. 



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

 midium'' stage, consists of a single row of flat cylindrical cells with 

 thick walls which frequently possess striations. Later on the cells 

 divide longitudinally and produce a thin expanded thallus, known 

 as the " Schizogonmm'' stage, which tapers to the base. The cells of 



Fig. 40. 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), formation of aplanospores in akinetes 

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

 Knebel.) 



the mature thallus are often arranged in fours and possess axile 

 stellate chloroplasts, whilst another feature is the presence of short 

 rhizoids that may occur in the stalk-like portion or else growing out 

 from the marginal cells. In the juvenile filament 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 margins. Sometimes the cells produce large, thick- 



