530 PHYSIOLOGY OF GROWTH. 



of the three leaflets). It is instructive to compare the anatomical 

 structure of the joint with that of the rest of the leaf-stalk. If we 

 examine transverse sections of the large pulvinus at the base of the 

 main leaf-stalk of Phaseolus, it is particularly striking that below 

 the epidermis, which is beset with hairs, occurs a very strongly 

 developed parenchyma, whose cells are almost the same in 

 character on the under and lower sides of the pulvinus. Fairly in 

 the middle of the section we observe a number of vascular bundles, 

 which surround the pith. On microscopical examination of a 

 transverse section from the middle of the main leafstalk, it is at 

 once seen that the tissue of the cortex, which in the pulvinus w r as 

 so well developed, is here only comparatively slightly developed ; 

 it occurs as collenchyma (in the projecting angles), and as the 

 ordinary cortical parenchyma. The vascular bundles surrounding 

 the pith are not aggregated in the middle of the section, but 

 lie more peripherally. The cells of the cortical parenchyma of 

 the pulvinus bring about the movements of variation of the leaves 

 in the bean, and also in Acacia, Mimosa, etc., and, as already 

 remarked, these movements are effected in the case of mature 

 organs, not by growth, but solely through varying conditions 

 of turgidity, the truth of which statement is at once obvious 

 when we consider that the pulvini maintain the size attained 

 on the completion of their growth, although for months they 

 bring about movements of the leaves. 



In order to prove that no processes of growth are exhibited 

 in connection with movements of variation, we make ink dots on 

 one flank of the pulvinus in the manner described in 153, and 

 repeatedly, for several days, measure the distance between them 

 by means of a horizontal microscope. Great care must be taken 

 that the pulvinus is always in the same position when the measure- 

 ment is made. 



The periodic movements of variation induced by change in the 

 conditions of illumination are due to the fact that the cortical 

 parenchyma of the pulvinus on two opposed sides does not vary 

 in turgidity and change in length to the same extent. Thus, 

 e.g., the evening depression of the leaflets of a bean leaf takes 

 place because the cells of the parenchyma of the upper side of the 

 pulvinus turgesce more vigorously than those of the antagonistic 

 side, so that a convex curvature of the upper side of the pulvinus 

 is brought about. 



One important factor, however, will not be obvious without the 



