216 DISEASES OF PLANTS. 



6th. The peduncles and petioles sometimes change into tendrils, 

 as in the vine ; this plant at first throws out many large leaves and 

 clusters of flowers ; but the food not being sufficient to support such 

 a profuse vegetation, the new leaves and clusters of flowers appear 

 smaller; the nourishment becoming still more scanty, at length 

 neither flower nor leaf is developed, and the peduncle and petiole 

 become tendrils, which, by attaching themselves to some firm bodies, 

 serve to sustain the rich fruit which is perfected on the lower parts 

 of the branch. 



7th. The last change we shall notice is the transformation of buds 

 into thorns. When a plant forms more buds than it can nourish, 

 some of them do not develop branches and leaves, but becoming 

 hardened by the accumulation of sap, which is insufficient for their 

 full perfection, they 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. 



Prickles, such as may be seen upon the rose, gooseberry, and other 

 plants, do not change by cultivation, for these are a natural appen- 

 dage, originating from the bark ; while the thorn may be found con- 

 nected with the wood, of which it seems to make a part. 



Diseases of Plants.* 



The diseases of plants (for these organized beings are, like ani- 

 mals, 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 

 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 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 deposite of car- 

 bon; 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 ab- 

 sorbs 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; there- 

 fore the absorption by the roots should be in proportion to the evap- 

 oration b)' the leaves. 



3d. External injuries often aflfect 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 favouring the descent of the cambi- 

 um, and promoting a more free circulation of the other juices. 



Smoke is injurious to plants, it being composed 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 



* This constitutes a department of Botany called patJwlogy ; a term derived from 

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



6th, peduncles and petioles become vines— 7fh. buds, how transformed— Prickles- 

 Diseases of plants— 1st, Constitutional— 2d, Light and heat— 3d, External injuries- - 

 Rains— Wind— Smoke. 



