212 LONGEVITY OF PLANTS. 



when hatched produce larvae, which, by their peculiar juices, 

 often rot the wood. . Of these insects called cynijps^ one kind 

 produces the hard protuberances on trees of ditt'erent kinds 

 which are called gall-nuts ; others which are softer and more 

 spongy are called apple-galls^ or berry -galls. Another kind ol 

 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 Cactus cochinillifer^ a Mexican 

 plant. 



5th. Diseases are po'oduced hy plants preying upon each other ^ 

 either Ijy fastening themselves %ipon 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; 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 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 ujDon 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 ; 

 Tust is chiefly confined to the grasses : both are of the fungi 

 famil3\ 



6th. Diseases resulting from age. Plants difier from animals 

 in one important circumstance, — the latter develoj? 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 ]3roduction 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 natural 

 constitution and the quantity of fruit which they produce. 



5th, Parasites — 6th, Diseases resulting from age. 



