Growth and Movement. 



123 



illustrate this. Figure 59 represents a leaf in its night posi- 

 tion; at a, b, and c (Fig. 60) are the motor organs. The posi- 

 tions of the motor organs of the leaflets are shown for the 

 nighttime at B, and for the daytime at A. Cross sections 

 of a petiole and of a motor organ are shown at C and D. 



It is seen that 

 while the petiole 

 is made rigid by 

 the disposition of 

 its vascular bun- 

 dles in a circle 

 outside the cen- 

 ter, as seen at C, 

 and by the wing- 

 like outgrowths 

 containing each 

 a vascular bundle 

 g, the vascular 

 bundles in the 

 motor organ are 



all thrown to the center; and surrounding them is a relatively 

 broad zone of thin-walled parenchymatous tissue which is 

 capable of undergoing great variation in the turgidity of 

 its cells. In the nighttime the upper portion of the paren- 

 chymatous tissue of the motor organ of the petiole becomes 

 less turgid than the under portion, and the leaf as a whole 

 rises in consequence ; but at the same time the motor or- 

 gans of the individual leaflets become less turgid on the 

 under side, and the leaflets are made to droop. In the 

 daytime the motor organs of the leaflets become less turgid 

 on their upper sides, and the leaflets rise in consequence, 

 while the motor organ of the petiole becomes less turgid on 

 its under side and the leaf as a whole drops to a lower plane. 



FIG. 59. 



Leaf of Scarlet Runner in its night position. 

 After SACHS. 



