142 PHYCOMYCETEAE 



recently proposed, are really valuable. Should zoospore formation in 

 Peronospora be confirmed it would seriously weaken the distinction be- 

 tween that genus and Pseudoper'onospora. 



The evolutionary tendencies within the Peronosporales are of interest 

 to students of phylogeny. The species of Pythium with long, narrow, 

 hypha-like zoosporangia would undoubtedly be included in the Sapro- 

 legniales were it not for those other species of Pythium with ovoid or 

 spherical zoosporangia which may even function as separable conidia, 

 thus forming a transition to Phytophthora. This genus still shows in some 

 species a close relationship to Pythium, in the production of submerged 

 zoosporangia and facultative saprophytic habits, while in other species 

 with well-developed conidiophores and strictly parasitic habits the genus 

 approaches closely the Peronosporaceae. The fact that the single egg in 

 the oogone is surrounded by periplasm, as in the Rhipidiaceae in the 

 Saprolegniales, would suggest that the relationship of the Pythiaceae is 

 closer to this family than to the Saprolegniaceae in which there is no 

 periplasm and the majority of species have numerous eggs in the oogone. 

 It has been suggested by some mycologists that from the Pythiaceae have 

 been derived on the one hand the Saprolegniales (through the Rhipi- 

 diaceae) and on the other hand the remainder of the Peronosporales. 

 As obligate parasitism became prevalent in the Peronosporales evolution 

 appears to have proceeded in several lines. The catenulate conidia of 

 Albugo call to mind the proliferating zoosporangia of some species of 

 Pythium as well as of Saprolegnia, while the sympodial conidiophores of 

 the more advanced species of Phytophthora remind one of the sympodial 

 branching in Achlya and some species of Pythium. The monopodia] 

 conidiophores of the Peronosporaceae do not resemble so closely any 

 structures in Pythium. As parasitism has progressed we also find the 

 transition from attached zoosporangia to separable zoosporangia (conidia) 

 leading finally to the conidium as found in Peronospora, in which zoospore 

 formation has been lost, although the plurinucleate condition persists. 



The fungi that have been considered in the preceding chapters of this 

 book have been largely aquatic in habit or reveal their aquatic ancestry 

 by producing naked flagellate cells in some stages of their development, 

 although in some genera these have been suppressed. This has been true 

 even for the majority of the strictly parasitic species of the Peronosporales 

 which have abandoned the aquatic habit to assume that of parasitism in 

 land plants. Besides the production of zoospores the great majority of 

 the foregoing organisms, except most of the Chytridiales series, show the 

 cellulose reaction promptly upon the application of chloriodide of zinc 

 solution. Many of those which fail to show this reaction promptly do so 

 when certain masking substances are removed. The presence of true chitin 

 is demonstrated for only a minority of the species. The forms with well- 



