S-APROLEGNIALES 149 



In several genera the sporangiospores encyst within the 

 sporangium, each spore enveloping itself in a thin cellulose 

 membrane. In other cases the spore is provided with a pair of 

 cilia, and escapes from the sporangium as a zoospore without 

 having first encysted. 



In the genus Aplanes the sporangiospores may encyst within 

 the sporangium, and then germinate in situ by germ tubes which 

 penetrate the sporangial wall and reach the outside. In Geo- 

 legnia the encysted sporangiospores are thick-walled and remain 

 quiescent in the sporangium until freed by the disintegration 

 of its wall. They then germinate by germ tubes, zoospores 

 being unknown. In Thraustotheca, Calyptralegnia, Dictyuchus, 

 and Brevilegnia the spore after encysting within the sporangium 

 germinates, usually by the emission of a biciliate zoospore which 

 swims about and finally encysts again before germinating by a 

 tube. In Thraustotheca, Calyptralegnia, and Brevilegnia the 

 encysted spores escape from the sporangium before germinating. 

 In Dictyuchus the zoospores on escaping from the cysts pass to 

 the outside through individual pores in the sporangial wall. 

 In Thraustotheca, Calytralegnia, and Brevilegnia the wall either 

 disintegrates or is ruptured by the expansion of the swelling 

 spores within; the spores escaping in a more or less indefinite 

 manner through an opening of indeterminate size. In all the 

 remaining genera of the family the spores, instead of encysting 

 within the sporangium, escape as biciliate zoospores through a 

 small pore at its tip. In Achlya and Aphmiomyces the escaping 

 spores reach the water just outside the mouth of the sporangium, 

 and there became encysted, each spore rounding up and assuming 

 a delicate membrane. The escaping spores thus form a more 

 or less spherical aggregation at the mouth of the sporangium. 

 Their failure to separate from one another on reaching the water 

 has been explained by Hartog as due to a mutual attraction, 

 perhaps chemical in nature, to which he applies the term adelpho- 

 taxy (Couch, 1924 a; Lounsbury, 1927). After a period of rest at 

 the mouth of the sporangium each spore slips from its membrane, 

 these being left behind as empty spheres. The escaping zoo- 

 spores are reniform (bean-shaped) and biciliate, the cilia arising 

 together at the lateral depression corresponding to the hilium 

 of the bean. After a period of swarming in which the zoospores 

 swim about actively they come to rest and encyst again, the 

 encysted spores being termed cystospores. They finally ger- 



