THE NERVOUS MECHANISM OF PLANTS 403 
Closer consideration, however, lessens the difference con- 
siderably. The motor mechanism of an animal is very 
largely either muscular or glandular. The vegetable 
cell seems to have much more in common with the gland 
cell of an animal than with its muscle. Stimulation of a 
nerve going to a gland frequently causes a flow of liquid from 
the latter, probably owing to a change in the permeability 
of the protoplasm of the gland cells. The contractile 
power is but little developed in vegetable protoplasm, and 
when present it seems to be rather passive than active, 
to be associated with recoil, rather than true contraction. 
Still, the latter is not entirely absent. We have seen that 
it can be detected in the pulsation of vacuoles, in ciliary 
motion, and in the crawling movements of the Myxomy- 
cetes. Its manifestation under an external stimulus seems 
to be evident when a filament of Mesocarpus splits up 
into its constituent cells as soon as an electric shock is sent 
through the water in which the plant is floating. 
Though the power of contraction is comparatively 
seldom found, the action of the gland cell is recalled by the 
power which vegetable protoplasm possesses of resisting or 
assisting the transit of water. The effect is really similar 
in both cases ; in the one the disturbance to the protoplasm 
leads to a contraction of its substance, in the other to its 
lessening its resistance to the passage of water through 
it. Each protoplasm responds in its own appropriate 
fashion, which is based upon the need of the organism of 
which it is part. The main requirement of most animals 
is freedom of locomotion or rapid assumption by the body 
of new positions. The most important duty of the plant is 
the regulation of the water supply upon which its con- 
stituent protoplasts are so dependent. 
The immediate result of such an increase of permea- 
bility is that the elastic recoil of the stretched cell mem- 
branes, which we have seen is a feature of every turgid celi, 
drives some of the water out of the cell, causing the latter 
to shrink in volume. 
