THE MO T EVENTS OF PLANTS. 



205 



about the center. (See ^| 164.) In figure 201, .1 and />' 

 show the contrast. If the leaf 

 be a compound one, there are 

 usually secondary motor or- 

 gans at the base of the leaf- 

 lets, as in the leaf of the bean 

 (fig. 202). Variation in the 

 turgor of the cells of the cor- 

 tex upon one side or the other 

 produces a sharp curvature of 



FIG. 201. FIG. 202. 



FIG. 201. Transverse sections through petiole of scarlet runner. A, through the rigid 

 portion; U, through the motor organ. Cr, g, vascular bundles; c, cortex; in, pith ; 

 r, deep channel along ventral side of petiole. Magnified about 10 diam. After Sachs. 



FIG. 202. Portion of a scarlet runner, which, originally growing erect, has been inverted 

 for several hours, resulting in geotropic curvatures of the primary motor organs P, P 1 , 

 P^. The lowest pair of leaves show secondary motor organs at the juncture of petiole 

 and blade. Similar ones are present in the upper compound leaves, but are not clearly 

 shown in the figure. The arrows show the position of the petioles when the plant was 

 first inverted. About two thirds natural size. After Sachs. 



the motor organ, which alters the position of the leaf or leaflet 

 (fig. 202). The concave surface of the motor organ is always 

 deeply wrinkled transversely, while the convex surface is 

 smooth. 



