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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 sutficient 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 thfy 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 naturahst. 
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 hght, 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 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 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 j it serves, therefore, to obstruct these 
* This constitutes a department of Botany called pathology ; a term derived from 
two Greek words, pathos^ disease, and logos, account of. _ 
6th, peduncles and petioles become vines— 7th. buds, how transformed— Prickles- 
Diseases of plants— 1st, Constitutional— 2d, Light and heat— 3d, Externalinjunes- ' 
Rains — Wind— Smoke. 
