THE POTATO DISEASE 249 



spot on the under side of the leaf. With a microscope 

 these threads are found to consist of simple or branched 

 conidiophores emerging singly or in clusters through 

 the stomata of the leaf, and rearing conidia or spores, 

 egg-shaped and colourless. The conidia produced on 

 the conidiophores which originate from the mycelium 

 of the fungus ramifying the tissues of the leaf, are thus 

 in a position, when mature, for ready dispersal by wind, 

 rain and other natural agents. A conidium thus dis- 

 persed may germinate at once, pushing a germ-tube, 

 or it may give origin to a number of zoospores when 

 alighting on a damp surface, as that of a Potato leaf 

 covered with water from rain or even dew. The 

 zoospores, furnished with hair-like appendages, move 

 about actively for some time in the water, and finally 

 settle down and emit a germ-tube, which enters the 

 tissue of the leaf through a stoma or directly bores 

 through the epidermis. 



Conidia washed by rain upon young tubers pro- 

 duce zoospores which infect the Potatoes, entering their 

 tissues, forming a mycelium and setting up decay, more 

 or less, unless, as it has been found to happen 

 frequently, "sweating" takes place after the Potatoes 

 are stored, when the mycelium spreads and converts 

 the mass, often aided by other micro-organisms, into 

 an ill-smelling state of putrefaction. Thus the fungus 

 may wear itself out, or be subdued by other organisms 

 more speedily resolving organic into inorganic matter. 

 But the mycelium of the fungus also passes down 

 diseased Potato stems, infects the tubers, and the 



