160 Dr. J. Burdon-Sanderson [June 9, 



gions — organs whicli, from their structure and position, can have no 

 other function. 



The action of the leaf to which the ph^nt owes its name and by 

 which it seizes its prey. is. in its general character, too well known 

 to require description. In the shortest possible terms, it is the 

 sudden change of the outer surface of each lobe of the leaf from 

 convex to concave, and at the same time the crossing of the two series 

 of marginal hairs, as the fingers cross when the hands are clasped. 

 ^Miat I desire to show with respect to it is. that here also the agents 

 are individual cells — that is, that the individual elements out of 

 which the whole structure is built are the immediate agents in the 

 production of the movement. 



A cross-section of the leaf shows the following facts : If the 

 section be made in the direction of the parallel fibro- vascular bundles 

 which run out from the mid-rib nearly at right angles, and happen to 

 include one of these bundles, it is seen that it consists of three parts, 

 viz. of the fibro-vascular bundle in the middle and equidistant from 

 both borders ; of the cylindrical cells of the parenchyma on either side, 

 and of an external and internal epidermis. The external epidermis 

 is smooth and glistening, and its cells possess thicker walls than those 

 on the opposite surface. 



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

 poeseseee the excitable hairs, three on each side, which in Dionasa 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 exclusivelv are excited bv touch. It is therefore of OToat 

 interest to know their structure 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 consists of minute 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 in- 

 distinguishable 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 

 (particularly 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 internal 

 surface of the lobe, suggests that although the parenchyma of the 

 inside has 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 



