THE MOVEMENTS OF PLANTS. l8l 



steles distributed about the center. (See Tf 136.) In figure 

 133, A and B show the contrast. If the leaf be a compound 

 one, there are usually secondary motor organs at the base of 

 the leaflets, as in the leaf of the bean (fig. 134). Variation 

 in the turgor of the cells of the cortex upon one side or the 

 other produces a sharp curvature of the motor organ, which 

 alters the position of the leaf or leaflet (fig. 134). The con- 

 cave surface of the motor organ becomes deeply wrinkled 

 transversely, while the convex surface is smooth. 



254. Spontaneous movements. — Only a few plants exhibit 

 spontaneous movements by means of motor organs. The 

 lateral leaflets of the telegraph plant (s, 

 fig. 13s), under normal conditions of 

 rather high temperature (about 32" C), 

 show jerky movements of such direction 

 that their tips describe an irregular el- 

 lipse, which is completed in i to 3 

 minutes. The leaflets of the clovers and 

 oxal is show much slower movements 

 described in the next paragraph. 



More commonly the turgor movements ^"'5-. i35--Leaf of Desmo- 

 are induced. The most common stimuli '^'"■J^ n*'""' sUe.-After 



bachs. 



are light and contact, although many 

 others suffice to induce them. 



255. Light movements. — Movements produced by the 

 variations of light have long been known as "sleep move- 

 ments." They are best observed upon the leaves of the 

 bean family, though many other plants exhibit them. Figure 

 136 shows the positions assumed by various leaves toward 

 nightfall. It will be seen that in compound leaves the leaf- 

 lets sometimes rise, so as to apply their outer faces to each 

 other ; others sink, so that the under surfaces are in contact; 

 others become folded in various ways. This position is main- 

 tained throughout the night. Upon the increase of light in 



