NERVOUS MECHANISMS 505 



to the existence of differentiated nerves. The way in which it is 

 carried ont in plants has been much debated, but since the dis- 

 covery of the continuity of the protoplasm through the cell-walls, 

 there is little doubt that we have here a similar mechanism. 

 There is scarcely any differentiation, but the power of the proto- 

 plasm to conduct disturbances from one part of the cell to another 

 is a matter of common observation. The connecting strands 

 between adjacent cells (fig. 1194) will suffice to suggest how 

 impulses from the tip of the root may reach the growing cells. 



The motor mechanism seems at first to be entirely ditierent 

 from that of the animal organism. Closer consideration, 

 however, lessens the difference considerably. The contractile 

 power is but little developed in vegetable protoplasm, but it 

 has its representative m the power 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 contrac- 

 tion 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. 



The plant shows, however, an almost complete absence of the 

 differentiation that reaches its highest point in the nerve-cell. 

 There is apparently no co-ordinating mechanism which receives 

 the impulses from the sense organs, and initiates in consequence 

 the resulting movement. This need not, however, lead us to 

 deny the existence of a nervous system, btit only to remark 

 upon the very slight degree to which it is differentiated. 

 Darwin has called attention to one instance in which something 

 of the kind is seen. When a tentacle of Drosera is stimulated, 

 the actual bending is preceded b}' a curious motility of the 

 protoplasm of the cells of its stalk which has been called 

 aggregation. If a tentacle on the leaf is excited, the tenta- 

 cles of the margin are gradually inflected towards the excited 

 spot, as already described. If the cells of one of the marginal 

 tentacles are watched during the experiment, their contents are 

 foimd to undergo this aggregation, but those nearest its apex 

 manifest it first. If the aggregation were the direct effect of 

 the stimulus, those which it reached first, i.e. those nearest 

 the base, would respond first. The stimulus, apparently, has to 

 travel up the tentacle to the gland, and a disturbance to originate 

 there in response, this disturbance travelling down the tentacle 

 in the direction of its base. Darwin has pointed out that this 

 corresponds m a measure to the reflex action of the animal 

 organism. 



