DISEASES OF PLANTS. 
211 
then exhibit the short, indurated process, called a thorn. It is 
said that wild plants by rich cultivation do in time become 
divested of their thorns, which change into what they seemed 
originallj 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 con- 
nected with the wood, of which it makes a part. 
324. 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 natural- 
ist. 1st. We notice constitutional diseases. Of this class are 
the varied hues 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. 2d. Plants become 
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, be- 
comes 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 retained for its 
nourishment ; therefore the absorption by the roots {endosmosis) 
should be in proportion to the evaporation by the leaves. 3d. 
External injuries often affect the health of plants. Rains in 
jure the wood by penetrating through apertures in the bark; 
the bark itself seems from its nature better fitted to bear the 
action of the weather. Winds,, when violent, are mechanically 
destructive to vegetables ; when moderate, the agitation which 
they produce is thought to be advantageous, by favoring the de- 
scent of the cambium, and promoting a more free circulation of 
the other juices. Smoke is injurious to plants, it being com- 
posed of particles which, though invisible to our sight, are yet 
too gross to be absorbed by the minute' pores of the leaves; it 
serves, therefore, to obstruct these pores, and prevent their ex- 
haling 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 dis- 
eases. Insects in particular make their way into the bark and 
external coats of the plant and deposit their eggs ; these eggs 
♦ Their study constitutes a department of Botany called pathology ; a term derived from two Greek 
A'ords, pathos, disease, and logos, account of. 
Prick.es.— 324. Diseases of plants— 1st, Constitutional— 2d, Excess or deficiency— 3d, External in 
Inriea-^Raius — Wiud — Smoke — 4th, Injuries from auimala. 
