yg THE ALGAE 



(a) Sessile sporangia that never become detached. These consist 

 of enlarged cells which develop in ahnost any position and 

 they produce biflagellate s warmers that may be isogametes. 



(h) Stalked terminal or lateral sporangia that are cut off from an 

 enlarged support cell which may give rise to several such 

 bodies. The apical portion swells out to form the sporangium 

 and cuts off a stalk cell underneath that frequently becomes 

 bent. The dividing septum possesses two ring-shaped cellu- 

 lose thickenings which may be connected with the detach- 

 ment of the sporangium when it is mature. The detached 

 sporangitmi is blown away and germinates under favourable 

 conditions to give bi- or quadriflagellate s warmers. 



(c) Fw?2n^/-shaped sporangia which are cut off at the apex of a 

 cylindrical cell, the outer wall splitting later at the septum, 

 thus liberating the sporangium, the subsequent fate of which 

 is not definitely known. The sessile and stalked sporangia 

 may occur on the same plant or else on separate plants. There 

 has been no cytological work to show whether there is any 

 alternation of generations and such an investigation would 

 be highly desirable. In one species, on the other hand, repro- 

 duction is wholly by means of aplanospores. 

 Rowland (1929) has investigated the physiology of the common- 

 est species, T. aurea, in some detail and he found that : 



(a) drought increases the resistance to plasmolysis ; 



(b) if the threads are dried first and then heated together with 

 cotton-wool and gelatine, the results suggest that the threads 

 hold water in a manner similar to that of cotton-wool, but 

 that the loss of water on heating is comparable to that ex- 

 perienced by a colloid or gel under the same circumstances 

 (cf.Fig.41); 



(c) in damp, warm weather only small cells are formed because 

 cell division is relatively rapid; 



(d) the threads can survive desiccation for at least six months ; 



(e) plasmolysis could only be produced in some of the cells by a 

 25 per cent solution of sea salt. 



In many respects, e.g. the heterotrichous nature of the thallus, 

 the different types of sporangia and the orange pigment, this alga 

 is strongly suggestive of the more primitive brown algae. This 

 feature, however, is discussed more fully in a later chapter (cf. 



p. 315)- 



