DISEASES OF CULTIVATED PLANTS 437 



vided into separate cells by divisions like those at the joints of a bam- 

 boo rod, as well as in the manner of spore formation and in the size, 

 color, form markings and structure of the spores themselves. It is 

 almost hopeless to undertake to illustrate types of spore production 

 and spore forms, since these are so varied and may differ so much 

 at different stages of the development of a single given species of 

 fungus, yet we may cite a few examples : 



Fungus spores may be produced as single spores or in naked 

 clusters attached to certain branches. We find this sort in the downy 

 mildew of the cucumber and its relative the peronospora of mustards ; 

 in potato early blight; in fruit rot of plum, cherry, peach, etc., and 

 later in the spores of apple scab. They may also be found in dense 

 clusters breaking through the skin of the plant like the many tubers 

 of a Dotato breaking through the earth-crust; such without further 

 conspicuous covering are found in the rust spots, in the anthracnoses 

 and the like. These dense clusters may arise beneath a special cover- 

 ing resembling nothing so much as the traditional beehive, but are 

 usually ejected forcibly from a specially provided opening at the top 

 of the cone or half-ball. A yet more interesting class is that in which 

 the spores are packed so many to a sac (usually eight) and a large 

 number of these crowded into a ball-like, hollow spore-case, such as 

 we find in black-knot, strawberry leaf-spot, the powdery mildews and 

 in some other instances. There is yet another sort in which the spore 

 sacs are abundant near the surface of the diseased part, as in leaf- 

 curl of the peach, where the maturity of the fungus is shown by the 

 change in color of the affected leaf surfaces. Other gradations will 

 be found as one proceeds in this study. 



THE SURVIVAL OP PARASITIC FUNGI. 



Further, respecting parasitic fungi we must realize that they are 

 all derived by specific processes of reproduction peculiar to the fungus 

 in question; in other words spontaneous generation does not find 

 support among the students of plant diseases. 



The presence of any given fungus leads us at once to infer the 

 previous existence, somewhere within reach, of a fungus of like spe- 

 cies from which this one was derived by definite methods of repro- 

 duction. Likewise, the destructive prevalence of a parasitic fungus 

 in any given time and at any given place, assures us of the necessary 

 supply of spores to start the trouble again under favorable conditions. 

 In fact, all our study leads us to look through mere phenomena, mere 

 evidences of disease, to find the specific parasitic growth which causes 

 them and the favoring conditions under which these develop. The 

 spores of fungi serve for them the same purpose as do the seeds in 

 higher plants; by reason of the extreme smallness of the spores they 

 are easily transported by the wind and become deposited like dust 

 particles upon exposed surfaces. Certain resting spores survive on the 

 fallen leaves or other parts and will be destroyed if these parts are 

 burned. (See Black-knot.) The survival of organisms capable of 

 infecting the new crop is certainly to be expected in plant diseases as 

 in epidemic disorders among people. 



