182 NEBRASKA STATE HORTICULTURAL. SOCIETY. 



ing things, they differ in certain things. For example the tree 

 has roots and leaves, while the horse has a mouth, a stomach, 

 and blood-vessels. The digested food of a horse is carried 

 rapidly to all parts of its body through a system of tubes, — the 

 arteries and veins, but in the apple tree the food of all kinds 

 merely soaks through the substance and has no tubes through 

 which it runs. But in this the apple tree is like some lower 

 animals, in which there is no system of arteries and veins, and 

 as a consequence the digested food passes from part to part by 

 a soaking process similar to that in the apple tree. All animals 

 having such a slow circulation are sluggish, some of them being 

 almost as slow and sluggish as the trees are. It is noticed that 

 just as animals are more active, their circulatory system is 

 more perfect. 



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 in a few minutes, but in 

 plants this is much slower on account of their want of a circu- 

 latory system. So it is with the 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 di- 

 seases are therefore slower and simpler than the correspond- 

 ing diseases of animals, and this fact must be taken into the ac- 

 count 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 fruit-grower should know something 

 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 can 

 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. 



