122 ELEMENTS OF BOTANY 
and very generally succeeds in capturing the fly or other 
insect which has sprung it. The imprisoned insect then 
dies and is digested, somewhat as in the case of those 
caught by the sundew, after which the trap reopens and is 
ready for fresh captures. 
141. Object of catching Animal Food.—It is easy to 
understand why a good many kinds of plants have taken 
to catching insects and absorbing the digested products. 
Carnivorous, or flesh-eating, plants belong usually to one 
of two classes as regards their place of growth: they are 
bog-plants or air-plants. In either case their roots find it 
difficult to secure much nitrogen-containing food, — that is, 
much food out of which proteid material can be built up. 
Animal food, being itself largely proteid, is admirably 
adapted to nourish the growing parts of plants, and those 
which could develop insect-catching powers would stand 
a far better chance to exist as air-plants or in the thin, 
watery soil of bogs than plants which had acquired no 
such resources. 
/ 142. Destruction of Plants by Animals. — All animals 
are supported directly or indirectly by plants. In some 
cases the animal secures its food without much damage 
to the plant on which it feeds. Browsing on the lower 
branches of a tree may do it little injury, and grazing 
animals, if not numerous, may not seriously harm the 
pasture on which they feed. Fruit-eating animals may 
even be of much service by dispersing seeds. But seed 
eating birds and quadrupeds, animals which, like the hog, 
dig up fleshy roots, rootstocks, tubers, or bulbs, and eat 
them, or animals which, like the sheep, graze so closely as 
to expose the roots of grasses or even of forest trees to be 
parched by the sun, destroy immense numbers of plants. 
