4 8 4 



NA TURE 



[Sept. 14, 1882 



The most remarkable feature of the internal surface is, that it 

 possesses the excitable hairs, three on each side, which in Dionaea 

 are the starting-points of the excitatory process whenever it is 

 stimulated by touch, as is normally the case when the leaf is 

 visited by insects ; for experiment shows that although the 

 whole of the leaf can be excited either by pressure or by the 

 passage of an induction current, the hairs exclusively are excited 

 by touch. It is therefore of great interest to know their struc- 

 ture and their relation to the excitable cells of the parenchyma, 

 with which they are in so remarkable a relation physiologically. 

 In sections such as that which we will now project on the screen 

 (Fig. 6), it is seen that each hair springs from a cushion which con- 



sists of minu'e nucleated cells inclosed by epidermis ; and that if 

 we follow this structure into the depth of the leaf, its central cells 

 gradually become larger, until they are indistinguishable from 

 those of the ordinary parenchyma of the leaf. By these cells it 

 must be admitted that the endowment of excitability is possessed 

 in a higher degree than by the ordinary cells of the parenchyma, 

 so that for a moment one is tempted to assign to them functions 

 corresponding to those of motor centres in animal structures 

 (pari icularly in the heart). There is, however, no reason for attri- 

 buting to them endowments which differ in kind from those we 

 have already assigned to the excitable plant cell. 



The fact that the excitable organs are exclusively on the 



f 



Fig. 



Fig. 



7r~~^ 10nKrl ' e; >f fi«d s:o as to prevent its closing. (From aphotograph). Fig. 8.— Diagram of ideal transverse section of lamina of leaf of Dionaea. 

 the needle inclosed in a circle represents the electrometer which in the experiment described was substituted for the galvanometer. On the 

 opposite side is shown ibe secondary coil of the inductorium. 111 is in connection with the capillary, / with the sulphuric acid of the electrometer. 

 Fig. 9. — Diagram of the pendulum-rheotome. k ly k-,, and £3 are the keys referred to. I and II. represent respectively the primary and secondary 

 coils of the inductorium. The leaf galvanometer, batter}-, &c, will be easily recognised. 



internal surface of the lobe, suggests that although the paren- 

 chyma of the inside ha< apparently the same structure, it has 

 not the same function as that of the outside— that is, that 

 although the cells of the outer layers are just like those of the 

 inner, they are either not excitable at all, or are so in a much 

 less degree. In this way only can we account for the bending 

 inwards of the lobe. In the unexcited state both layers air 

 equally turgid ; as the effect of excitation the internal layers 

 become limp, the external remaining tense and distended. 



I will now endeavour to illustrate the motions of the leaf 

 by projecting them on the screen. Here are several leaves 

 which have been prepared by attaching one of the lobes to a 

 cork support ; the other is free, but a very small concave minor 



has been attached to its external surface near the margin. The 

 image of the light which falls on the mirror is reflected on the 

 wall behind me. In this way the slightest movement of the 

 lobe is displayed. By this contrivance I wish to show you two 

 things — first that a very appreciable time elapses between the 

 excitati n and the mechanical effect ; and secondly, that when 

 the leaf is subjected to a series of very gentle excitations, the 

 effects accumulate until the leaf chses. This we hope to show 

 rig down a camel hair pencil several times in succession 

 on a sensitive hair, doing it so deftly that at the first touch the lobe 

 will scarcely move at all. At each successive touch it will bend 

 more than at the preceding one, until you see the lever suddenly 

 rise, indicating that the leaf has closed. The purpose which I have 



