DISEASES OF PLANTS. 31 



an excess of light, so as to cause the escape of too much oxygen, or 

 too rapid a deposite of carbon, are also causes of disease. By a 

 knowledge of the properties and characteristics of plants we may per- 

 ceive their wants and frequently apply remedies adapted to their dis- 

 eased condition. Their health is often affected by external injuries. 

 Rains and winds also injure them, oftentimes. Smoke obstructs the 

 pores of the leaves and is thereby greatly prejudicial. 



Animals are a frequent cause of disease in plants. Some penetrate 

 the bark and deposite their eggs, producing larvae and the insect 

 cynips. By one kind of these, protuberances are produced, as the nut- 

 gall of oaks, apple or berry galls. Some prey on the juices, as with 

 the insect cochineal, a species of which is so valuable for dying a 

 scarlet color. The Mexican plant cactus cochinilifer is that which 

 they feed upon. Disease is likewise produced by contiguity with 

 other plants, either by ejecting deleterious matter from their roots or 

 withdrawing that which is necessary for one or the other. Mosses 

 and lichens attach themselves to trees and absorb moisture or attract 

 insects, both of which destroy the wood : they do not however feed 

 on the juices, and are therefore called false parasites. The miseltoe 

 pierces the bark and feeds on the juices and is a true parasite. An- 

 other parasite called the pterospora is found on the leaves and 

 branches of trees. Smut and rot are fungi, the former fastening itself 

 on the ears of cereal grains and the latter preying on the seeds. If 

 these seeds be planted the disease will be continued in the plant. Rust 

 and ergot are also fungi, the one a disease of rye and the other of 

 grasses. As plants renew their parts annually, they seem less liable 

 to be affected by old age ; still their powers of renewal, or vital prin- 

 ciple, becomes exhausted in time as with animals. In annual plants, 

 the production and maturing of fruit exhausts their energies, during 

 the year, and in biennials, in two years. These, however, as with 

 perennials, depend much on their constitution and the amount of their 

 fruit, as with the apple tree which, being very fruitful, does not often 

 attain to so great an age as the oak, the fruit of which is light. 



The effects produced by insects on plants is vastly greater than in 

 producing deformities. Like great fires, however, they may often 

 prove a benefit and maintain a balance among the various species of 

 plants, for the devastating effects of these insignificant agents are 

 wonderful. Scarcely a plant is without one or more species of insect. 

 The diseases they produce often constitute an important article of 

 food, medicine and commerce, as we have said, in the cactus, or cochi- 

 neal insect, the lac insect, the cantharia or Spanish fly, the gall apples, 

 and the nut galls. 



The sweeping destruction produced by the locust affords a striking 

 discrepancy between the magnitude of the means and that of the effects. 

 They can strip entirely of their foliage thousands of square miles of 



