Chap.X.] nature op the MOTOR IMPULSE. 209 



certain cells, opposed as long as they are alive and not irri- 

 tated by the expanded state of their contents. 



A somewhat different view has been advanced by other 

 physiologists namely that the protoplasm, when irritated, 

 contracts like the soft sarcode of the muscles of animals. 

 In Drosera the fluid within the cells of the tentacles at the 

 bending place appears \inder the microscope thin and homo- 

 geneous, and after aggregation consists of small, soft masses 

 of matter, undergoing incessant changes of form and float- 

 ing in almost colourless fluid. These masses are completely 

 redissolved when the tentacles re-expand. Now it seems 

 scarcely possible that such matter should have any direct 

 mechanical power; but if through some molecular change it 

 were to occupy less space than it did before, no doubt the 

 cell-walls would close up and contract. But in this case it 

 might be exi)ected that the walls would exhibit wrinkles, 

 and none could ever be seen. Moreover, the contents 

 of all the cells seem to be of exactly the same nature, 

 both before and after the aggregation ; and yet only a few of 

 the basal cells contract, the rest of the tentacle remaining 

 straight. 



A third view maintained by some physiologists, though 

 rejected by most others, is that the whole cell, including the 

 walls, actively contracts. If the walls are composed solely 

 of non-nitrogenous cellulose, this view is highly improbable; 

 but it can hardly be doubted that they must be permeated 

 by proteid matter, at least whilst they are growing. . Nor 

 does there seem any inherent improbability in the cell-walls 

 of Drosera contracting, considering their high state of or- 

 ganisation; as shown in the case of the glands by their 

 power of absorption and secretion, and by being exquisitely 

 sensitive so as to be affected by the pressure of the most 

 minute particles. The cell-walls of the pedicels also allow 

 various impulses to pass through them, inducing movement, 

 increased secretion and aggregation. On the whole the be- 

 lief that the walls of certain cells contract, some of their 

 contained fluid being at the same time forced outwards, per- 

 haps accords best with the observed facts. If this view is 

 rejected, the next most probable one is that the fluid contents 

 of the cells shrink, owing to a change in their molecular 

 state, with the consequent closing in of the walls. Anyhow, 



