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DISEASES OF PLANTS. 
buds into thorns. When a plant forms more buds than it can 
nourish, some of them do not develope branches and leaves, but 
becoming hardened by the accumulation of sap which is insuf- 
ficient for their full perfection. They exhibit the short indurated 
process called a thorn. It is said that wild plants by rich cul- 
tivation, do in time become divested of their thorns, which 
change into what they seemed originally destined for, viz. leaves 
and branches. 
Prickles, such as may be seen upon the rose gooseberry and 
other plants, do not change by cultivation, for these are a natural 
appendage, originating from the bark ; while the thorn may be 
found connected with the wood, of which it seems to make a 
part. 
DISEASES OF PLANTS.* 
The diseases of plants, for these organized beings, are like 
animals, subject to disease and death, may in many cases arise 
from causes within the knowledge of the attentive naturalist. 
1st. We notice constitutional diseases. Of this class are the 
varied colors 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 color of the leaf. 
2nd. Plants seem diseased by being subjected to too great or 
too scanty a supply of food, as light, heat, water, air and soil. Ex- 
cess of light causes an escape of oxygen, and a too rapid depo- 
sit 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 diminishing 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 re- 
tained 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 mechanically 
distructive to vegetables ; when moderate, the agitation which 
they produce is thought to be advantageous, by favoring the 
* This constitutes a department of Botany called pathology, a term deri- 
ved from two Greek words pathos disease, and logos account. 
Thorns and prickles — Diseases of plants — How produced — Effects of 
light, heat, water, air and soil — External injuries. 
