The Life of the Plant. | 235, | 
berant growth of the parenchyma, while the leaves are pro- 
duced in ordinary numbers, but with scarcely any paren- 
chyma between their fibro-vascular bundles, causing the 
misshapen branch to appear as if covered with moss. ‘The 
highest form of development of these structures is seen in 
the true gad/s, the production of which is due to punctures 
made in the leaves, flowers, fruits, buds, and young bark 
by the female gall-wasp, belonging to various species of 
Cyntps, which then lays its eggs in the wound. ‘The various 
kinds of gall are most abundant on the oak-tree, on which the 
total number of species that have been observed is upwards _ 
of 150. The best known is the common oak-gal/, pro- 
duced by the puncture of Cyzips scutel/aris on the under side 
of the leaf (Fig. 379).! As morbid phenomena galls are 
usually of but small significance. Whether they are regarded 
as the nidus of a number of useful insects, or, many of them, 
as articles of considerable value in commerce, their usefulness 
greatly exceeds the injury which they inflict on the plant. — 
It would occupy too much space to enumerate those — ea 
animals which are hurtful to the plant by the external in- 
juries inflicted by feeding on them, &c. The part which 
insects play in this respect is well known to every observer 
of nature. | | 
AGE AND DEATH OF PLANTS. 
If the life of a plant is not shorterted by any disease, 
and if no animal or man appropriates it to its own purpose, 
gathers it, and kills it, its term of life is still usually limited. 
Some die after they have once produced fruit [monocarpic 
plants], one or two years [anual or biennial plants] being 
required for their development; while others [ polycarpic 
plants] produce fruit many times and live for many years, 
attaining sometimes an enormous age. ‘The silver-fir sel- 
* [Oak-spangles are smaller, flat bodies, produced also on the under — 
oe of oak-leaves by the attacks of an insect, Diplolepis lenticularis. 
—ED.] 
