212 
LONGEVITY OF PLANTS. 
when hatchea produce larvse, which, bj their peculiar juices, 
often rot the wood. Of these insects called cynvpa^ one kind 
produces the hard protuberances on trees of different kinds 
which are called gall-nuts ; others which are softer and more 
spongy are called apjjle-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 dye- 
ing ; this species feeds on the Caches cocldnilUfer^ a Mexican 
plant. 
5th. Diseases are produced hy plants preying upon each other ^ 
either hy fastening themselves upon their surfaces^ or hy 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, false and true parasites j the 
former adhere to the plant without feeding on its juices, as 
mosses and lichens ; they derive their nourishment from the 
atmosphere, but they injure the tree by harboring insects and 
attracting moisture which often rots the part of the stem on 
which they grow. The mistleto is a true parasite whose root, 
piercing the bark of trees, plants itself in the albiirnum, 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 frequent- 
ly 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 fungous 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 to rye ; 
7'ust 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 
obstructed, until they at length decay from old age. Plants on 
the contrary renew themselves every year; that is, they form 
new vessels 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 appear to be in perennial plants any prescribed term of 
existence. The production of fruit exhausts the vital energy of 
the plant, in annuals in one year, in biennials in two, in peren- 
nials in a longer or shorter period according to their natviral* 
constitution and the quantity of fruit which they produce. 
Furasites — 6th, Diseases resulting from age. 
