i De Se a ere The Life of the Plant Goo nas 
which are injurious as weeds, depriving other plants of light 
and nourishment and therefore of the most essential con- 
ditions of life, or themselves stunting them in their growth 
by climbing over or twining round them, and dragging them 
to the ground. | 
Reference must finally be made to epiphytes, or those 
plants which only grow on other plants without deriving 
nutriment from their tissues. Plants of this class which > 
occur in our zone, for example, many Mosses and Lichens 
[and some Ferns], can scarcely be considered as morbifi-  —_ 
cants. 
The injuries which plants receive from animals are ex- 
tremely numerous and various. When the injury is to the 
root, it results in an insufficient supply of nutriment to the 
plant; when, on the other hand, it is the leaves that suffer, = > 
the absorbed food-materials are not assimilated in sufficient 
quantity, and the plant suffers from weakness or exhaustion. — 
Injuries to the bark cause ‘bleeding,’ gangrene, or decay. — 
There are also a number of animals that live in or on plants, 
which, without visibly injuring them externally, cause an 
irritation in the parts of the tissue affected which results in | 
morbid phenomena. It is common, for example, to see the © 
leaves of the hop, cabbage, and other plants, crumpled up| 
into blisters and rendered unhealthy by the attacks of - 
some species of aphis on the under side. The form: of 
these abnormal productions is usually so characteristic that 
it is possible from them to determine the animal by which 
they are caused, and wece versa. ‘Vhus, aphides, which lay 
their eggs in the plant, cause blister-like swellings, in 
which the larve live until the period of their metamor- 
phosis, at the expense of the plant. The structures which 
they produce, and which are generally not injurious, are 
sufficiently well known in the lime, hazel, elm, and other 
trees and shrubs. W2llow-galls and rose-galls are a somewhat 
higher form of these bodies. ‘The former are produced 
especially on willows by the puncture of the Cecadomyia 
