DISEASES OF PLANTS. 
273 
descent of the cambium, and promoting a more free circulation 
of the other juices. 
Smoke is injurious to plants, it being composed of particles, 
which although invisible to our sight, are yet too gross to be 
absorbed by the minute pores of the leaves ; it serves therefore, 
to obstruct the*pores, and prevent their exhaling the oxygen gas 
Vhich 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 
when hatched produce larvae which often by their peculiar jui- 
ces rot the wood. These insects are called cynips. One kind 
produces the hard protuberances on trees of different kinds, which 
are called gall-nuts, or nut-galls; others which are softer and more 
spongy are called apple-galls or berry-galls. Another kind of 
insect, called cochineal attaches itself to the bark of trees, and 
preys upon the juices. One species of the cochineal is of a 
brilliant scarlet color and much valued for its use in dyeing ; 
this species feeds on the Cactus opuntia, a Mexican plant. 
5th. Diseases are produced by plants preying upon each 
other, either by fastening themselves upon their surfaces, or by 
so near a location as to deprive others of their necessary food. 
Parasites fasten themselves upon the surfaces of other plants ; 
they are distinguished into two kinds, the false and true par- 
asites ; the former adheres to the plant without feeding on its 
juices, as mosses and lichens. These derive their nourishment 
from the atmosphere, but they injure the tree by harboring in- 
insects and by attracting moisture, which often rots the part of 
the stem on which they grow. The misseltoe is a true parasite 
whose root piercing the bark of trees, plants itself in the albur- 
num, and absorbs food from it, in the same manner as if were 
fixed in the soil. The Pterospora is a very curious parasite 
which is sometimes found upon the leaves of shrubs :* but more 
frequently upon the branches and leaves of trees. Mushrooms 
are of the class of false parasites. Smut is a black fungus, 
which fastens itself upon the ears of oats and other grain. The 
rot is a fungus excrescence which preys upon the seed ; if seeds 
which have this disease fastened upon them are sown, the rot 
will be propagated also. Ergot is a disease mostly confined 
* A species of this genus was found in the woods east of Troy, upon the 
leaf of the Vaccinium. The color of the whole plant, consisting of two 
flowers a and kind of leaf, was that of a red rose. 
Smoke — Insects —Plants injurious to each other — Parasites. — ■ 
