I 



ORDER PERONOSPORALES 129 



into the periplasm which surrounds the centrally placed egg except in the 

 rare cases in which two to six eggs are produced. In P. polysporum Poitras 

 (1949) nearly one fourth of the oogones produce from two to six oospores. 

 The antherid is also plurinucleate at first but only one nucleus functions in 

 fertilization of each egg. The filament bearing the terminal antherid may 

 arise at a great distance from the oogone ("diclinous") or from near by 

 on the hypha bearing it ("monoclinous"). In some cases the antherid is a 

 small segment of the hypha immediately under the oogone, the conjugation 

 tube entering the latter through the base ("hypogynous"). In a few 

 species the antheridial filament wraps in a spiral around the hypha bear- 

 ing the oogone. Often the oospore develops parthenogenetically. Some- 

 times many antherids may become attached to the same oogone although 

 only one antherid functions in its fertilization. Through a conjugation 

 tube one male nucleus is introduced into the egg which then forms a 

 thick wall and becomes an oospore. The oospore may completely fill the 

 oogone ("plerotic") or may leave a space between it and the oogone wall 

 ("aplerotic"). It is usually thick-walled and smooth but may be reticu- 

 lately thickened externally. Germination may be delayed for a long 

 while. It is effected by the production of a long germ ^be or of a short 

 germ tube terminating in a zoosporangium, or in some cases the oospore 

 may produce the zoospores internally, emptying them through a short 

 beak or exit tube into a vesicle as in the germination of a conidium 

 (Drechsler, 1947). (Fig. 40.) 



Asexual reproduction is mostly by means of zoospores. Typically the 

 zoosporangium forms a beak (usually short but sometimes several times 

 as long as the diameter of the zoosporangium). The tip of the beak softens 

 and out of it flows the protoplasm of the zoosporangium, to form a spher- 

 ical mass, the scncalled vesicle. Within this the differentiation into zoo- 

 spores is completed and the kidney-shaped biflagellate zoospores rupture 

 the plasma membrane of the vesicle and escape singly, or several in a 

 clump w^hich separate subsequently. After swimming for a while the zoo- 

 spore encysts and germinates by a germ tube or, as apparently first 

 reported by Cornu (1872), sends out a short papilla from whose apical 

 opening emerges a single zoospore, with or without a vesicle, or sometimes 

 a vesicle in which are produced several zoospores. The process may be 

 repeated three or four times, the zoospores of the successive crops being 

 smaller each time. 



Three more or less distinctive types of zoosporangium may be dis- 

 tinguished as follows: 



1. Slender filaments, simple or branched, of the same size and appear- 

 ance as the vegetative mycelium, and opening at the tip or tips of the 

 hyphae to form a short or long emission tube at whose apex a vesicle is 

 formed. This filamentous zoosporangium may be separated from the re- 



