PYTHIUM 



103 



ends of short branches of hyphse that sprout from the 

 host plant into the open air (see Fig. 33). There is 

 a beak at the apex of each sporangium; this beak 

 swells, and becomes a sac for the reception of the whole 

 protoplasmic contents of the sporangium (Fig. 33, B). 

 The contents of the sac split up 

 into a number of unwalled cells, 

 and each of these cells becomes 

 an active zoospore furnished 

 with a pair of cilia. These de- 

 velopments can occur only when 

 there is sufficient water to cover 

 the sporangia. The zoospores 

 swim about for a short time, 

 after which they cease to move ; 

 in germination they develop 

 hyphse which, when opportunity 

 favours, will carry on the vicious 

 parasitism of their kind. If 

 water is not sufficient, the 

 sporangia of P. Baryanum, 

 although prevented from pro- 

 ducing zoospores, are not de- 

 feated ; they overcome the 

 difficulty by immediately grow- 

 ing into new hyphae. Indeed, 

 the sporangium, in this event, 

 becomes an asexual spore, 

 termed, in botanical parlance, 

 a " conidium " (Gr. Jconis, dust) 

 determination to live and multiply displays much 

 versatility and incidentally gives us an insight into the 



Fig. 33. — Pythium. 



A, Branch of the mycelium with 

 three sporangia (s), x 125; 

 B, sporangium (s) discharging 

 contents (b), which have be- 

 come zoospores, x 145; C, 

 an oospore (osp) producing 

 an asexual sporangium (s), 

 x 300. 



Thus Pythium in its 



