112 POTATO-FUNGUS. 



them soon dies. It then withers and falls to the ground ; the dead 

 tissues, and often ulso the membrane of the ooj^onia, decay, and the 

 oospores alone survive. In places where they exist in great numbers 

 they can be detected in the ground. A considerable period of appa- 

 rent inactivity precedes their germination ; it does not, in the cases 

 hitherto observed, take place till after the winter has passed, no matter 

 whether the maturity has been arrived at in autumn, or, as is mostly 

 the case, in early summer. The germination of the oospore and the 

 penetration of the young Fungus into tlie host-plant occur in the moist 

 and warm season which succeeds the winter ; the renewal of summer 

 vegetation is taken for granted. The oospores therefore act as the 

 organs of hibernation in the life of Peronosporea: ; and for the species 

 which grow only during the summer, the oospores are the only means 

 for carrying them through the winter. Others may, as has been 

 shown above, hibernate by means of perennial mycelium. 



All the foregoing statements have been confirmed by direct obser- 

 vation made in special cases. In no particular do they rest on mere 

 conjecture. It will be apparent then that, as regards the whole life- 

 history of the Peronosporece, there exists a general plan, which, how- 

 ever, presents numerous variations in detail according to the species. 

 This must be kept clearly in mind in the investigation of every case 

 which occurs in practice, so that, while we follow the general well- 

 established rule, we must, nevertheless, not expect identical pheno- 

 mena in all cases. 



Of the Saprolegniem, which must be considered in connection with 

 the question before us, only a few spegies of Pythium are parasites. 

 Most of the species live in dead organic bodies. What we know of the 

 species parasitic on living plants corresponds with the .known phe- 

 nomena of the Peronosporece, to such an extent, at any rate, that it is 

 unnecessary to describe them here minutely. One fact only is impor- 

 tant, viz., that the oospores of the Saproleyniea often live through the 

 winter, or, at least, are able to remain a considerable time in appa- 

 rent inactivity ; while, on the other hand, it has been observed that 

 they are capable of germinating within a few weeks, or even a few 

 days, from the time they attain maturity. From this it is evident 

 that oospores, even of closely related forms, do not always pass 

 through the same series of changes ; and, since the oospores of some 

 species of Peronospora present great similarity of structure to those of 

 Saprolegniecc, it may be regarded as possible that such species would 

 exhibit the same phenomena in their history. 



There is yet another circumstance which must here be referred to. 

 There are parasites which criunot complete their entire development in 

 one host-plant, but rec^uire two dilferent species of hosts ; in the one 

 they complete a distinct stage of their development, in tlie other 

 the second stage, and then return to tlie first host to begin again. 

 This necessary change of the host is called hetercccia or mekccia. We 

 have familiar examples of this among animals in the Tapeworms, and 

 among Fungi in many of the llusts. It should be remarked hei'e that 

 metoecia has not been observed, and is not even likely, to occur in any 

 Peronospora or Saprolcjnia ; all the species belonging to these families 

 wliich have been thoroughly investigated, are known to have only a 

 single host on whicli they complete without change their entire 



