208 



THE DOWNY MILDEWS 



the gametes is perhaps due to their exposure to atmospheric 

 conditions, as would be the case when growing upon terrestrial 

 or floating organic matter. The lack of water for the transport 

 of the male gametes is nicely met by a tubular outgrowth of 

 the antheridium which penetrates the oogonium when it ruptures, 

 discharging the male close to the female gametes (Fig. 130, D, 

 an) . In the majority of the species, singularly enough, the female 

 gametes germinate without being fertilized. The gametospore 

 germinates as in Vaucheria. 



It should be stated that, in related forms, large multiciliate 

 zoospores, like those of Vaucheria, are formed and also motile 

 male gametes. It is also important to note that there are several 

 parasitic algae closely related to these alga-like fungi. Their 

 parasitic habit and partial loss of chlorophyll clearly indicate that 

 they are in a transition state from the algae to the fungi. It is 

 therefore safe to state that in this group we have evidence of 

 the derivation of the fungi from the algae. 



Fig. 131. Hyphae of Peronospora extending through the tissues of a plant 

 and absorbing food from the cells by means of haustoria, h. 



79. Order b. Peronosporales. Downy Mildews and White 

 Rusts. — These forms are parasites upon the higher plants espe- 

 cially and include two of the most destructive fungi, the potato 

 and the grape vine blight. Phytophthora infestans causes the 

 potato rot. This disease was widespread in the eastern United 

 States in 1901 causing the entire loss of the crop in some sections. 

 It would be difficult to imagine a pest more perfectly adapted to a 

 destructive career. Let us begin the life history with the germi- 

 nation of a spore which has fallen upon a leaf. This spore forms 



