1882.] on the Excitability of Plants. 155 



resistent cushion consists of cells, that is, of little bladders, each of 

 ■which is distended with liquid ; and its tendency to expand as a 

 whole is due to the tendency to expand of the innumerable cells of 

 which it is made up). In the unmutilated state, these are squeezed 

 into a smaller space than that which they would assume if they were 

 left to themselves ; and, consequently, as their expansion is j)revented, 

 or curbed on one side, it acts on the opposite side, so as to bend the 

 cylinder in the direction of the restraint. 



All of this we can, perhaps, better understand by a model ; and it 

 is possible to make one which, not only in form, but in principle, 

 corresponds to the living mechanism it is intended to illustrate. In 

 the model the axial bundle is represented by a strip of leather, the 

 innumerable cells of the excitable cushion by an india-rubber bag. 

 By a pump we are able to fill this cell or cushion more or less with 

 fluid, and thus to vary its tension, and you see that if we increase the 

 tension, the stem rises. Bv diminution it suddenly falls, just as the 

 Mimosa leaf does when irritated. 



We have come then to this point — that the reason why the leaf 

 suddenly sinks on excitation is that the cells undergo a sudden 

 diminution of tension or expansion. But our inquiry is not yet 

 terminated. We have still to ask — How is this loss of tension 

 effected '? The answer is, by discharge of water. In the unexcited 

 state all these cells are distended or charged with liquid. Suddenly, 

 when the structure is excited, they let out or discharge that liquid, 

 and it finds its way first into the inter-cellular air spaces, and secondly, 

 out of the motor organ altogether. This we know to be a fact by an 

 experiment of Pfeffer's, which must be regarded as one of the most 

 important relating to the mechanism of plants that was ever made. 

 He observed that if the leaf-stalk is cut ofi:' from the motor orsan, a 

 drop of fluid ajDpears at the cut surface at the moment that the latter 

 bends downwards on excitation, and that in the experiment described 

 just now, in which the upper part of the motor organ is cut oft', there 

 is also, so to speak, a sweating of liquid from the cut surface. 



We are, therefore, certain that liquid escapes, but why does it 

 escape ? That I shall explain farther on, and will now proceed to 

 two other examples. One is a plant which is a great favourite in 

 London, for it is one which flourishes even in London smoke — 

 Mimulus. For our purpose it is good chiefly because its structure is 

 very simple. It is one of those examj^les in which excitability is 

 associated with the function of fertilisation, and inasmuch as this is 

 a very transitory purpose, the proj^erty itself is transitory. When 

 the cells of the stigmatic surface are touched they discharge their 

 liquid contents, and consequently become limp. The outer layer of 

 the lip is elastic, and tends to bend inwards. Consequently when the 

 inner cells lose their elastic resilience it is able to act, and the lip 

 bends inwards. In another allied plant, Goldfussia anisojyln/Ua 

 (Fig. 3), which was described forty years ago by the Belgian 

 naturalist Morren, we have the same mechanism. In this plant, as 



