4io 



BOTANY OF THE LIVING PLANT 



shoots which grow out from the tubers. There may be also other 

 sources of reinfection. The control measures to be taken are to destroy 

 by lire all infected haulms and leaves, to avoid carefully the use of 

 tainted " seed " tubers ; and, as a preventive, to spray the young 

 growing crop with suitable disinfectants, especially if the season is 

 wet in the middle summer. But a more hopeful line of prevention is 

 by the use of " immune varieties," which are able to resist the attack 

 of the parasite. 



Fig. 310. 



Fertilisation of the Peronosporales. 1. Peronospora parasitica, young multi- 

 nucleate oogonium (og ), and antheridium (an). 2. Cystopus Candida. Oogonium with 

 the central, uninucleate egg {os), and the fertilising tube (a) of the antheridium which 

 introduces the male nucleus. 3. The same. The fertilised egg (0) surrounded by 

 periplasm (p). (After Wager, x 666.) (From Strasburger.) 



No mention has been made of sexual reproduction in the Potato 

 Fungus. As a matter of fact sexual organs have not been proved to 

 exist in Phytophthora infestans under normal conditions of life. Like 

 some other plants it seems to be able to propagate itself indefinitely 

 without the recurrence of sexual reproduction. But under special 

 conditions of saprophytic culture the sexual organs have been 

 produced, though it is still doubtful whether they are ever formed 

 when the Potato Fungus is growing on the living host. In other 

 members of the Peronosporales the details which have been frequently 

 observed show a striking parallelism with those of Vaucheria. The 

 sexual organs have been found in Peronospora and Cystopus to be 

 formed within the host-plant (Fig. 310). The oogonium appears 

 as a spherical swelling on the end of a hypha, while a thinner branch, 



