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 contractile 

 power is but little developed in vegetable protoplasm, and 

 when present it seems to be rather passive than active, 

 to produce frequently 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 cmwjing_jn^ej^ 

 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, it has its representative in 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 modifying 

 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 constituent protoplasts are 

 so dependent. 



The effects of stimulation may be seen in glandular 

 organs in plants as well as animals. Both Drosera and 

 Dioncea are excited by contact to pour out on to the surface 

 of their leaves acid digestive secretions, which are the 

 result of changes in the activity of the gland-cells. 



The conduction of the stimuli received is due in 

 animals to the existence of differentiated nerves. The way 

 in which it is carried out by plants has been much debated, 

 but since the discovery of the continuity of the proto- 

 plasm through the cell -walls there is little doubt that we 



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