630 



LECTURE XXXVI. 



nevertheless only passive, and is due to the curvature of the motile organs. Fig. 

 367 may serve to illustrate the true process more exactly; after the removal of the 

 laminae there remain only the three motile organs of the leaflets on the common 

 leaf-stalk, as represented. In A these are very decidedly in the diurnal position : 

 the leaflets are no longer all extended in one plane, but are more upright ; at B 

 these motile organs have assumed their nocturnal position. It is hardly necessary to 

 add that as the light varies, and from its after-effects, these organs may assume all 

 possible positions and curvatures between those of A and B. 



The coarser anatomical structure is illustrated in Fig. 368. C is a transverse 

 section through the rigid portion of the leaf-stalk itself; as in most other stiff petioles 

 which are channelled above, there are several vascular bundles G arranged 

 approximately in a circle, in addition to two more slender ones {g) running in the 

 edges of the channel; the remaining tissues are green cortex c, and soft pith 7«. The 

 appearance is quite otherwise in the transverse section D of the motile organ, 



Fig. 367.— Upper portion of the leaf-stalk of the 

 Scarlet Runner, with the three motile organs of 

 the leaflets. -4 diurnal position ; B nocturnal 

 position. 



Fig. 368.— C transverse section through 

 the rigid portion of the leaf-stalk of the Scarlet 

 Runner. D transverse section of a motile 

 organ (slightly magnified). 



although it is fundamentally nothing more than a modified portion of the leaf-stalk 

 itself. The cortex c is here much more strongly developed, and consists of very succu- 

 lent, strongly turgescent, parenchyma, a little thicker on the lower side than on the 

 upper, though otherwise there is no important difference on the two sides. It is im- 

 portant to mention this point, since in this tissue we have the active motile substance 

 of the organ, and we shall afterwards see that the movements and curvatures of the 

 latter depend essentially only upon a different reaction of the upper and lower paren- 

 chyma c c towards variations in the light. Here again, therefore, the matter depends 

 not on visible relations of organisation, by means of which irritability is to be explained, 

 but on invisible molecular structure ; however it must by no means be forgotten that in 

 discussing the mechanics of the movements themselves, the coarser structural relations 

 are also to be kept in mind. In the middle of this mass of active parenchymatous 

 tissue runs a strand G, which, on being highly magnified, is at once seen to be com- 

 posed of a large number of fused vascular bundles ; it has to be supposed that the 



