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 

 originally destined for, viz., leaves and branches. Pricldes^ 

 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 ; wdiile the thorn may be found con- 

 nected with the wood, of which it makes a part. 



324. The diseases of jylants"^ (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 Imies 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 supjyly 

 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 saj), 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 j^roduces 

 dropsy, and the ]3lant 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. Bains 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. SmoJce 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 w^hich is necessary for the decomposition 

 of the carbonic acid, and the consequent deposition of carbon. 



Ith. 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 Botanj' called patholo^ij ; a term derived from two Greek 

 words, pathos, disease, and logos, account of. 



Prickles.— 324. Diseases of plants— 1st, Constitutional— 2d, Excess or deficiency— 3d, External in* 

 juries*— Raiua— Wind— Smoko—4tli, Injuries from aoimala. 



