634 LECTURE XXXVI. 



illumination, when either the lower or the upper side of the swelling parenchyma 

 is much expanded. 



Fig. 372 may afford an approximate, though not an exact idea of the changes 

 in dimensions of this parenchyma in the alternating diurnal and nocturnal positions. 

 It represents a longitudinal section through the uppermost portion of the common 

 leaf-stalk, on which, to the left, is situated one of the three motile organs, with the 

 mid-rib // of the corresponding leaflet. Beneath the epidermis p the parenchyma 

 of the petiole as well as of the mid-rib consists of large thin-walled cells; in the 

 centre the vascular bundles G G ascend in the petiole, with the pith m between 

 them ; but as soon as they enter the motile organ, they close together and form an 

 axial strand devoid of pith, which eventually enters the lamina of the leaflet 

 to form the mid-rib with its lateral ribs. In making this preparation, care was 

 taken to make the section somewhat thicker on the lower side of the motile 

 organ (at b b) than on the upper side, where the plate is very thin : hence the 

 turgescence of the lower side and its tendency to expand were increased (the 

 preparation lying in water), and those of the lower side lessened. The organ 

 has, therefore, become curved upwards, just as if it were in the diurnal position, 

 and the thicker lower side b b presses the axial strand, by means of its tendency 

 to expand, concavely upwards; and this pressure is at the same time so strong, 

 that the swelling tissue of the upper side is very strongly compressed, as seen. 

 If the section is made so that the swelling tissue of the upper side remains thicker 

 than that of the lower side, a preparation is obtained which represents approximately 

 the nocturnal position : the swelling tissue of the upper side, when the preparation 

 lies in water, is then expanded, that of the lower side compressed. 



I think these somewhat prolix descriptions will sufficiently prepare the reader 

 to understand what now follows. I shall here rely partly on my own older 

 observations, but chiefly on a detailed investigation by Pfeffer. 



Proceeding first then from the easily demonstrated fact that by darkening an entire 

 plant or even one leaf only, the nocturnal position results after from half an hour to an 

 hour, and when this has occurred the diurnal position is restored on the subsequent 

 access of light ; the next question is naturally. What changes in the swelling tissue call 

 forth these two antagonisdc movements ? Of course it is at once clear that in a down- 

 ward curvature the upper side must become longer than the lower, and that in an 

 upward curvature the contrary must take place. Provided that a fully developed motile 

 organ is under consideration, it may be established by observation and calculation 

 that even after numerous up and down curvatures no permanent elongation of the 

 entire organ, or, what amounts to the same thing, of the axial strand, results. From 

 this, then, it is to be concluded that when one side of the organ has become convex, i.e. 

 longer than the other, or again relatively shorter when the curvature is in the opposite 

 direction, a corresponding increase or decrease of the water in the half of the organ 

 concerned must have taken place, because no other change whatever is conceivable. 

 For the present we may leave out of account the question how this is accomplished, and 

 use this conclusion simply to obtain an expression for the mechanics of the movement 

 itself, for we may now say, as darkness comes on, that side of the organ which is to become 

 convex in the first place obtains more water — i. e. its turgescence and its volume 

 increase and become greater than on the opposite side. This is induced by the change 



