DISEASES OF PLANTS. 133 



Viewed in this light, the subject is very practical and bears 

 directly upon the interests of agriculture. Primarily the object of 

 farming is the production of those plants which afford clothing and 

 nutriment to man and food for domestic animals. Plants, which 

 from any cause fail in the accomplishment of this object and are 

 hence of less value, may be said to be diseased. 



This being plain, I will now ask you to bear in mind that there 

 is a remarkable analogy between the diseases of plants and those 

 of animals. Thus we may have plant-maladies which come upon 

 whole districts as epidemics come upon whole communities ; we 

 have those which spring up among plants of a certain locality and 

 which reappear year after year from local causes just as we have 

 endemic diseases among men. We may have general diseases 

 affecting the whole plant, and special diseases affecting only a 

 part. As we shall soon see, we have diseases of plants which 

 closely resemble diseases afiecting man and the domestic animals, 

 and in some cases the causes of the diseases seem to be absolutely 

 identical. 



Before commencing the study of either the general or the special 

 diseases of plants, we must review the elementary facts in vegeta- 

 ble anatomy. These dry details are, by no means, interesting in 

 themselves. The recital of them is, on the contrary, dull, and it 

 will tax your patience ; but at the risk of tiring you, I must notice 

 some of the more important features. 



We shall study first the anatomy of a flowering plant, and I 

 shall select for this purpose the ordinary buckwheat. It consists 

 of roots, stem, leaves and flowers. When a piece of this plant is 

 picked in midsummer and carefully examined in the laboratory, it 

 is found to consist largely of water. The water gives it roundness 

 and freshness. Its amount can be accurately estimated by com- 

 paring weights of the plant before and after drying. The 

 percentage of water in the buckwheat is TO per cent. Having 

 dried the plant and driven off 70 parts of water in every hundred, 

 we may proceed in two ways to determine the other constituents, 

 by burning the tissue and saving all the products of combustion. 

 One of these ways consists in heating the plant in a vessel of glass 

 without allowing the air to have access to it. Upon the applica- 

 tion of heat, the plant is seen to give off slight fumes of vapor, and 

 to rapidly darken in color, until at last only a mass of unconsumed 

 carbon is left in our retort. The carbon retains the form of 

 the dried plant, just as charcoal preserves the shape of the wood 



