1882.] on the Exdtahilitij of Plants. 159 



extreme simplicity, is a better subject of investigation with reference 

 to this question, than any other. Each filament is a ribbon consisting 

 of (1) a single fibro-vascular bundle, (2) delicate cells of regular 

 cylindrical form, (3) an epidermis of somewhat thick-walled cells. 

 [Microscopical preparations were shown.] In Mimosa we saw that 

 the epidermis and vascular bundle took only a i3assive part in the 

 production of the motion. Here, the part they play is even less 

 important. Everything depends on the parenchyma, which, when 

 excited, shrinks by discharging its water. Pfefier proved this by 

 cutting off the anther tube from the filaments, and then observing 

 that on excitation a drop collected on the cut surface, which was 

 reabsorbed as the filament again became arched. It is obvious that 

 if the whole parenchyma discharges its liquid, each cell must do the 

 same, for it is made up entirely of cells. To understand how each 

 cell acts, we have only to consider its structure. Each consists of 

 two parts — an external sac or vesicle, which is of cellulose, and, so 

 long as the cell is in the natural or unexcited state, over-distended^ 

 so that, by virtue of its elasticity, it presses on the contents with 

 considerable force ; and secondly, of an internal more actively living 

 membrane of protoplasm, of which the mechanical function is, so long 

 as it is in its active condition, to charge itself fuller and fuller with 

 liquid — the limit to further distension being the elastic envelope in 

 which it is enclosed. In this way the two (the elastic envelope and 

 the protoplasmic lining) are constantly in antagonism, the tendency 

 of the former being towards discharge, that of the latter towards 

 charge. This being so, our explanation of the effect of excitation on 

 the individual cell amounts to this — that the envelope undergoes no 

 change whatever, but that the j)rotoplasm lining suddenly loses its 

 water-absorbing power, so that the elastic force of the envelope at 

 once comes into play and squeezes out the cell-contents. Con- 

 sequently, although here, as everywhere, the protoplasm is the seat of 

 the primary change, the mechanical agent of the motion is not the 

 protoplasm, but the elastic envelope in which it is enclosed. 



The complete knowledge we have gained, from our study of the 

 anther filaments of Centaurea, of the mechanism of the excitable j)lant- 

 cell, can be applied to every other known example of irrito-contrac- 

 tility in the organs of plants, and particularly to that most remark- 

 able of all such structures, the leaf of Dioncea muscipida. Although 

 • I described the structure of the leaf just eight years ago in this 

 room, I will occupy a moment in repeating the description. The 

 blade of the leaf is united on to the stalk by a little cylindrical 

 joint. Here are two models, in one of which the leaf is represented 

 in its closed state, in the other in which it is in its unexcited or ojdcu 

 state. The leaf is everywhere contractile — that is, excitable by 

 transmission, but not everywhere susceptible of direct excitation — or, 

 in common language, sensitive. It is j^i'ovided w^ith special organs, 

 of w^hich w^e do not find the counterpart in any of the plants to 

 which reference has been made, for the reception of external impres- 



