LONGEVITY OF PLANTS. 217 



pores, and prevent their exhaling the oxygen gas which 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 larvos, which, by their peculiar juices, often rot the wood. 

 These insects are called cynips. One kind produces the hard pro- 

 tuberances on ti"ees 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 brilliant scarlet colour and much 

 valued for its use in dying ; this species feeds on the Cactus cochinil- 

 lifer, 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 necessaiy food. Parasites fasten them- 

 selves upon the surfaces of other plants ; they are distinguished into 

 two kinds, the false and true parasites ; the former adhere 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 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 false 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 Teneriffe is the Dracaena draco, 

 which, according to many circumstances, appears to have some 

 thousand years of age. In England, at Blenheim Park, it is said, 



4lli, Animals— 5th, Parasites— 6th, Diseases resulting from age— Aged treea. 

 ]9 



