228 AGRICULTURAL EXPERIMENT STATION. [Jan. 



reported the trouble as one quite new to them. They stated 

 that it may show itself on any part of the plant and spread 

 ra])idly over the W'hole, so that all the leaves are practicall}" 

 destroyed in two or three days. In spreading from plant 

 to plant, they observe, it seems somewhat erratic, attacking 

 one here and another there, Avithout order, but eventually 

 taking all in the bench. It will be seen that this is a very 

 serious aftection where conditions favor, and one of nuich 

 im}iortance, from the extent of its ravages and the rapidity 

 wnth which it does its w^ork. 



It was impossible to recognize the presence of any fungus 

 with the unaided eye, and the fact of such presence was at 

 lirst doubted. But microscopic examination demonstrated 

 the presence on all diseased leaves, even in the earliest 

 stages, of the mycelium and spore threads of a fungus of the 

 imperfect form germs Cladospormm . The mycelium of the 

 fungus (figs. 25 and 26, oil) grows freely within the leaf 

 tissue, and after it is well developed sends branches to the 

 surface through the pores (stomata) of the lower surface of 

 the leaf. Frequently several of these threads come to the 

 surface through a single pore. Just at the mouth of the 

 pore there is usually formed a closely packed mass of small 

 cells, wdiich originates from the threads, but whose precise 

 development I have not followed. This may be called the 

 hyphal Jcnot (fig. 26, a), and is ordinarily large enough to 

 conceal, nearly or wdiolly, the guard cells of the pore. From 

 this knot arises a cluster of few or several erect spore threads 

 (fig. 25). These threads, at first simple, may remain so 

 indefinitely, cutting oft' the simple spores (fig, 25, sp.) from 

 tlunr ends ; or thoy may l)ranch, producing at their ends 

 short cells Avith all the characteristics of spores, which may 

 remain attached to the threads and undergo further dcA^'elop- 

 ment. This results in the most highly complicated form of 

 the spore threads, and consists in the successive acropetal 

 production of sprout buds from the originally terminal joints, 

 so that there are formed chains of successively smaller and 

 smaller cells, producing a much branched and very complex 

 appearance (fig. 2i)). All the cells thus produced appear to 

 be functional spores, juid the rai)id spread of the disease in 

 the greenhouse shows how promptly they are capa])le of 



