DISEASES OF PLANTS. 299 



the varied colours of some leaves, such as the box and holly ; 

 this is supposed to be owing to certain juices which by changing 

 their elements, vary the colour of the leaf. 



2d. Plants seem diseased by being subjected to too great or 

 too scanty a supply of food, as light, heat, water, air and soil. 

 Excess of light causes an escape of oxygen, and a too rapid 

 deposit of carbon ; the sap, incapable of sustaining so great a 

 degree of action, becomes exhausted, the plant withers, and 

 the leaves fall off. In this situation the food should be either 

 increased by watering or the vegetation retarded by diminish, 

 ing the light. Excess of heat absorbs the juices of the plant ; 

 deficiency of heat produces dropsy and the plant losing its 

 leaves ultimately decomposes. More water is evaporated by a 

 plant than is retained for its nourishment ; therefore the more 

 that is absorbed by the roots, the more should be evaporated 

 by the leaves. 



3d. External injuries often affect the health of plants. 

 Rains injure the wood by penetrating through apertures in the 

 bark. The bark seems from its nature better fitted to bear 

 the action of the weather. Winds when violent are mechan- 

 ically destructive to vegetables ; when moderate, the agitation 

 which they produce is thought to be advantageous, by favouring 

 the descent of the cambium, and promoting a more free circu- 

 lation of the other juices. 



Smoke is injurious to plants, it being composed of particles, 

 which although invisible to our sight, are yet too gross to be 

 absorbed by the minute pores of the leaves ; it serves there- 

 fore, to obstruct the pores, and prevent their exhaling the 

 oxygen gas which is necessary for the decomposition of the 

 carbonic acid and the consequent deposition of carbon. 



4th. Plants sustain injuries from animals which produce 

 diseases. Insects in particular make their way into the bark 

 and external coats of the plant and deposit their eggs ; these 

 eggs when hatched produce larva?, which often, by their pecu- 

 liar juices, rot the wood. These insects are called cynips. 

 One kind produces the hard protuberances on trees of different 

 kinds, which are called gall-nuts, or nut-galls ; others which 

 are softer and more spongy are called apple-galls or berry, 

 galls. Another kind of insect, called cochineal, attaches itself 

 to the bark of trees, and preys upon the juices. One species 

 of the cochineal is of a brilliant scarlet colour and much valued 

 for its use in dyeing ; this species feeds on the Cactus opuntia, 

 a Mexican plant. 



5th. Diseases are produced by plants preying upon each 



2d. Light and heat 3d. External injuries Rains Winds Smoke 4th. 

 Animals. 



