THE NEEVOUS 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 cell, 

 drives some of the water out of the cell, causing the latter 

 to shrink in volume. 



