THE DISEASES OF PLANTS. 89 



will pass from one person to another. It is so with the diseases 

 of plants. If potato " rot" gets a start it will go from potato to 

 potato until all are affected. 



If we allow smut to stay in the corn field it will spread. If 

 a cherry or plum tree has black-knot the disease will soon at- 

 tack the other trees until all are killed or nearly killed, and no 

 good fruit results. This point, therefore, we should firmly fix 

 in our minds, that whenever disease appears upon a plant we 

 should first of all try to prevent it from spreading by destroying 

 the diseased part, or, if necessary, the whole plant. And there 

 is only one effective way of destroying disease in plants, and 

 that is by burning. It will not do to cut off a black-knot limb 

 from a cherry tree and throw it in the fence corner or on the 

 brush heap. The disease will spread from the cut off branch. 

 It should be burned up. So with the peach tree affected by 

 "the yellows." Once the disease has started it is useless to try 

 to cure it or to remedy it. The diseased tree or plant or the 

 diseased part should be destroyed. But we can prevent it from 

 spreading, if we take action in time. Substances and methods 

 used for preventing the spread of the disease are called "pre- 

 ventives." 



NATURE OF DISEASE. If we begin with a giant oak or 

 white pine and arrange the plants known to us in order of size 

 down to the smallest grass plant, only a couple of inches high, 

 or the still smaller moss, we shall take in a great many plants, 

 but not all. There are very many others still smaller and 

 much simpler in their form and mode of growth. Perhaps 

 you have observed the greyish lichen growing on the old 

 fence rails or on the side of a boulder. It is not much thicker 

 perhaps than this paper and yet it is a kind of plant it is 

 one of the lower orders of plants. Then you have seen the 

 blue mold or fungus on the side of a cheese, it also is a low form 

 of plant life. The smut growing in the ear of corn, the 

 rot of the potato, the rust of wheat, and the other forms of 

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