SPOT, OR SPOT DISEASE. 165 



parent, in which case the marginal ring almost 

 disappears, or else loses all its green, remaining a 

 pale buff and retaining its marginal ring. Often 

 several adjacent spots unite and form larger 

 ones, but the centers of the uniting spots always 

 remain distinct. Spots which have become en- 

 tirely transparent except at the center may be in- 

 cluded in the enlarging spots, remaining visible 

 as transparent areas in the large buff spot. From 

 the appearance of the small, sunken areas in the 

 center of the spots, many erroneous views as to 

 the relation of insects to the disease have arisen. 



A few days of damp, cloudy weather will cause 

 the development of reproductive bodies upon 

 many of the spots. These appear to the naked 

 eye as innumerable blackish, tiny, hair-like points. 

 Each spot is capable of producing thousands of 

 the spores, and each spore is able, under the proper 

 conditions, to germinate and grow, and in so doing 

 infect a healthy leaf or a healthy portion of the same 

 leaf. The spores, as a matter of fact, are wafted 

 about in the air and are constantly settling down 

 on the plants, where they only await favorable 

 conditions to grow into the leaf and produce other 

 spots. We have made experiments which show 

 that in ten hours of one night, under ordinary 

 conditions existing in a greenhouse, fifty to sixty 

 of these spores will settle on a space three inches 

 in diameter. Every spore is able to produce a 

 spot, and the only reason that they do not do so 



