197 
ing acted on by diastase, and thus rendered soluble, that it can 
be taken up by the sap and used as food in the building up of 
new tissues. The failure to secrete sufficient diastase thus causes 
a condition quite comparable to that of severe indigestion in man 
or the higher animals. 
By far the greater number of plant diseases are caused by the 
action of parasites. The number of kinds of parasites that infest 
plants is very great. Probably no plant of economic importance 
is free from them, and the more widely cultivated crops have to 
contend with a formidable number of parasitic foes. These may 
be either animal or vegetable, and they belong to widely differing 
groups. In the vegetable kingdom plant parasites are found 
among the slime moulds, the bacteria, the green algae, the fungi, 
and a few even among the flowering plants. In the animal king- 
dom they are less widely scattered, being found only among the 
nematode worms, the mites and the insects. It is inthe great 
group of chlorophylless plants called fungi that we find by far 
the greatest number of plant parasites. The diseases known as 
smuts, rusts, mildews, leaf-spots and moulds are all caused by 
fungus selon while many of the blights, rots and wilts are 
also due to t 
All parts i ae plant are liable to be invaded by parasites. 
Roots, stems, leaves, flowers and fruits each have their special 
enemies. he surface only may be the point of attack, or the 
parasite may burrow deeply in the tissues. The nature of the 
injury caused will depend on the habit and structure of the host 
plant, on the point of attack, and on the character of the parasite. 
In some cases it may be little more than the loss of a certain 
amount of food material, the host and the parasite being so ad- 
justed to each other that the latter lives with a minimum of in- 
convenience to the former. Plants of wheat or oats infested by 
smut show very little inconvenience from the presence of the my- 
celium of the former in their tissues. It is only at maturity when, 
instead of ripened grain, we find the black powdery masses of fun- 
gus spores that the extent of the injury is suspected. Such cases, 
however, are rare. There are usually secondary complications 
that do far more harm than the mere loss of food. Thus the 
