a teacher's knowledge of plant pathology. 191 
J 
The slow circulation in plants implies slow progress in their 
diseases. In an animal a poison may be carried to all parts of 
the body in a few seconds or at most a few minutes, but in 
plants this is much slower on account of their want of a circula¬ 
tory system. So it is with bacterial diseases which may pervade 
the whole body of the animal in a short time, but in the plant 
their dispersal through the body is very slow. Plant diseases 
are therefore slower and simpler than the corresponding diseases 
of animals, and this fact must be taken into the account by 
the person who wishes to know even a very little about plant 
pathology. 
Sources and Causes of Plant Diseases. 
In the second place the teacher of botany should know some¬ 
thing as to the more common sources and causes of plant diseases. 
These are briefly as follows: 
1. Thirst, which in plants as in animals is the condition of 
the body in which there is a deficiency of water. Plants need 
water for the same purposes as animals, the principal one of 
which is to keep the inner tissues wet enough so that they may 
do their work properly. But both plants and animals lose a good 
deal of water, and this loss is especially great in dry air, and for 
this reason there must be an extra amount to make good this 
loss. 
2. Starvation, which is like that of animals, namely, the lack 
of a sufficient amount or kind of food. We know that to do 
well a domestic animal must have what we call a “ balanced 
ration," that is, one in which all the constituents which are neces¬ 
sary to build up all parts of the body are present in proper 
proportions. Now it is just so with a plant: it must have a bal¬ 
anced ration also in order to do best, and if any of the necessary 
food constituents are wanting it will suffer from starvation. 
Plants growing out of doors never starve for the food constitu¬ 
ents which come from the air, but they frequently suffer for 
want of some which they should get from the soil. Now and 
then a soil is deficient in nitrogen or phosphorus, or perhaps 
iron, and the plant suffers. In many regions there is a deficiency 
of potash or lime, or of still other constituents. 
