LONGEVITY OF PLANTS. 
217 
pores, and prevent their exhaling the oxygen gas whicli is necessary 
for the decomposition of the carbonic acid and the consequent de- 
position of carbon. 
4th. Plants sustain injuries from animals^ which produce diseases. 
Insects in particular make their way into the bark and external coats 
of the plant and deposite their eggs; these eggs when hatched pro- 
duce larvae, which, by their pecuhar juices, often rot the wood. 
These insects are called cynips. One kind produces the hard pro- 
tuberances 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^ at- 
taches itself to the bark of trees, and preys upon the juices. One 
species of the cochineal is of a brilhant scarlet colour and much 
valued for its use in dying ; this species feeds on the Cactus cochinil' 
lifer ^ a Mexican plant. 
5th. Di" ases 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 them- 
selves upon the surfaces of other plants; they are distinguished into 
two kinds, the false and true parasites j the former adhere to the 
plant without feeding on its juices, as mosses and hchens. These 
derive their nourishment from the atmosphere ; but they injure 
the tree by harbouring insects, and attracting moisture which often 
rots the part of the stem on which they grow. The mistletoe is a 
true parasite, whose root, piercing the bark of trees, plants itself in 
the alburnum, and absorbs food from it, in the same manner as if it 
were fixed in the soil. The Pterospora is a very curious parasite 
which is sometimes found upon the leaves of shrubs, but more fre- 
quently upon the branches and leaves of trees. Mushrooms are of 
the class of falsp parasites. Smut is a black fungus, which fastens itself 
upon the ears of oats and other grain. The rot is a fungus ex- 
crescence which preys upon the seed ; if seeds which have this dis- 
ease fastened upon them are sown, the rot will be propagated also. 
Ergot is a disease mostly confined to rye. Rust is chiefly confined 
to the grasses ; both are of the fungi family. 
6th. Diseases resulting from age. Plants differ from animals in 
one important circumstance; the latter develop their organs at 
once ; these organs in process of time become indurated and ob- 
structed, until they at length decay from old age. Plants, on the 
contrary, renew themselves every year ; that is, they form new ves- 
sels to convey the juices, new leaves to elaborate them, and new buds 
to produce flowers and fruits. Plants do not, then, like animals, seem 
destined to die with old age ; or there does not seem to be in peren- 
nial plants any prescribed term of existence. The producing of 
fruit appears to exhaust the vital energy of the plant, in annuals in 
one year, in biennials in two, in perennials in a longer or shorter 
period, according to their natural constitution, and the quantity of 
fruit which they produce. Apple-trees, which bear heavy loads of 
fruit, are very short-lived in comparison with the oak, which per- 
fects from each flower but one of six seeds, and this fruit is but a 
small acorn. 
There are some trees now known to exist, which are supposed to 
be of great age ; in the Island of Teneriflfe is the DRAOjaNA draco, 
which, according to many circumstances, appears to have some 
thousand years of age. In England, at Blenheim Park, it is said, 
4th, Animals— 5th, Parasites— 6th, Diseases resulting from age— Aged trees. 
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