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IRRITABILITY.—-CARNIVOROUS PLANTS. 863 
tive and negative geotropism are frequently used carelessly, the 
qualifying expressions positive and negative being frequently 
omitted, Darwin adopts the term heliotropism in the sense of 
bending towards the light ; apheliotropism for the contrary direc- 
tion, i.e. away from the source of illumination ; and, in the same 
manner, geotropism to imply towards the earth, and apogeo- 
tropism for bending in opposition to gravity, or from the centre 
of the earth. 
In addition to the foregoing terms, diaheliotropism is some- 
times used to express 2 position more or less transverse to the 
light which induced it ; and diageotropism to a similar position 
with regard to the radius of the earth. 
Irritability.—It has been already stated that some move- 
ments of plants are dependent upon the agency of insects. But 
though it has long been known that insects thus induce 
movements in certain 
plants, such as Drose- 
ra, Dionxa, Nepenthes, 
&c., it is only by the 
observations of Dar- 
win, Miiller, Hooker, 
Vines, Riess, Wills, 
and many others, that 
we have learnt that the 
insects, which by these 
movements are caught, 
serve for nutrition, 
being dissolved and 
absorbed. It has been 
also demonstrated that 
this solution of nitro- 
genous matters is due 
to the presence of a 
kind of ferment which 
closely resembles that 
of the peptic glands of Fig. 1175. Leaf of Drosera, showing a Fly caught by 
animals. It has like- the incurved glands. (After J. W. Groves.) 
wise been proved that 
this ferment is only efficient when associated with an acid ; 
and hence this solution is a true digestive process like that of 
animals. During the solution and absorption of these nitro- 
genous matters the protoplasm retracts from the walls of the 
cells in the form of a ball. In fig. 1175 is shown a leaf of 
Drosera (Sundew) where some of the glands have bent over 
and caught an insect. Such plants are now commonly termed 
carnivorous. 
Another remarkable instance of a carnivorous plant is the 
Utricularia, upon the leaves of which little pouches or air-sacs 
(fig. 1176), are developed. These sacs have a somewhat elabo- 
