a suitable power of the microscope, the fine white branching 

 stalks may be plainly seen growing out of the breathing pores 

 or stomata in the epidermis, sometimes one and sometimes sev- 

 eral growing from a single opening ; on the sides and tips 

 of their branches will be seen little white egg-shaped bodies ; 

 these are the spores or seed of the fungus causing the blight. 

 If the interior of a leaf be examined there will be found run- 

 ning in all directions among the cells, especially those of the 

 under side of the leaf, fine white threads which are the myce- 

 lium or body of the fungous plant. These threads absorb from 

 the cells among which they run the nutriment which has been 

 elaborated for the use of the potato itself, and cause them to 

 die and shrivel up or decay. The spores as soon as they are 

 mature are very readily broken off from their stalks, and being 

 so light are easily borne by the wind to healthy vines where 

 they germinate in any moisture they may find upon the leaves 

 or stems and make their w^ay through the skin or epidermis in- 

 to the succulent tissues beneath, where the threads develop and 

 in their turn send out a crop of spores which help to spread the 

 disease. 



If the weather is sufficiently warm and damp this develop- 

 ment and spread is very rapid, so that a large field of apparently 

 healthy vines may be entirely killed in a very few days. If this 

 destruction of the tops occurs early, the tubers cannot mature 

 and the yield will be a light one and of inferior quality ; more- 

 over, the fungus is not confined to the tops but makes its way 

 into the tubers also. The potatoes thus attacked are very likely 

 to decay, and great loss occurs, not only before they are dug but 

 even after they are stored in the cellar, if the conditions there 

 should be favorable. The threads of the fungus live over winter 

 in the tubers which, if used for seed, serve to start the disease 

 again the following summer. 



Figure i represents a magnified section of a potato leaf affected 

 with the blight. The leaf like all parts of the plant is made up 

 of cells of various sizes and shapes. The upper and lower surfaces 

 are covered by single layers of cells called epidermis shown at a 

 and d. The upper side of the leaf consists of elongated cells 

 arranged perpendicularly to the surface and closely packed 



