t 796 ] 



of these threads comes in contact with a leaf-cell the latter 

 gets discoloured and putrefied. Occasionally a thread comes 

 out at the upper surface also (vide Fig. 74 a). The stomata 

 getting blocked up with mycelia, transpiration is prevented, and 

 putrefaction hastens. Conidiophores, /. e., jointed branches 

 bearing little fruit-like bodies (conidia), appear chiefly at the 

 under surface. The conidia observed under a high power mi- 

 croscope magnifying about 250 diameters will be found to be 

 divided into compartments; and if a conidium is placed on 

 a moist substance, each compartment will be found coming 

 out as a ciliated zoospore and sailing about in the slightest 

 film of moisture. These zoospores after a little time become 

 non-motile and more spherical. After resting awhile each 

 zoospore, if properly conditioned, throws "out a mycelium 

 and thus the life of the parasite is repeated. 



1.366. The conidia also sometimes throw out pro-my- 

 celium without producing zoospores. The conidia are 

 carried about with the wind, and the zoospores swim over the 

 surface of leaves bedewed with moisture, and the infection 

 spreads in this two-fold way. Insects and birds also act 

 as carriers of infection from field to field. When the 

 mycelia reach the tubers, they decompose the cells and 

 corrode the starch. In bad cases the tubers rot altogether, 

 but in mild cases the mycelium hibernates in the tuber and 

 becomes perennial and these tubers which contain the disease 

 in an undeveloped form may give rise, when they are used 

 as seed, to potato-rot in the next crop. But perennial 

 mycelium cannot survive an unusual amount of heat, cold 

 or moisture, and diseased seed-potatoes, therefore, do not 

 necessarily produce a diseased crop. 



1.367. Besides the non-sexual reproductive functions by 

 means of mycelia and conidia (which usually form zoospores 

 as an intermediate stage before the pro-mycelial growth), 

 peronospora infestans is reproduced by sexual means also. 

 If a section of the leaf intersected by mycelial growth is 



