206 PLANT LIFE. 



296. Spontaneous movements. — Only a few plants exhibit 

 spontaneous movements through the motor organs. The lat- 

 eral leaflets of the telegraph plant (s, fig. 203), under normal 



conditions of rather high temperature (at 

 least 22 C), show jerky movements of 

 such direction that their tips describe an 

 irregular ellipse, which is completed in 

 1 to 3 minutes. The leaflets of the 

 clovers and oxalis show much slower 

 movements (of a few hours period), 

 which are usually obscured by the light 

 movements described in the next para- 



Fig. 203. — Leaf of Hesmo- ____t, 

 diu.l gyrans. Two graph. 



Sa , c r hs natural slze ~ After More commonly the turgor movements 

 are induced. The most common stimuli are light and con- 

 tact, although many others suffice to induce them. 



297. Photeolic movements. — Movements produced by the 

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

 ments ;" more properly, photeolic movements — that is, move- 

 ments induced by variation of light. They are best observed 

 upon the leaves of the bean family, though many other plants 

 exhibit them. Figure 204 shows the positions assumed by 

 various leaves toward nightfall. It will be seen that in 

 compound leaves the leaflets sometimes rise, so as to apply 

 their outer faces to each other ; others sink, so that the un- 

 der surfaces are in contact ; others become folded in various 

 ways. This position is maintained throughout the night. 

 Upon the increase of light in the morning, the day position 

 is assumed. The cutting off of light artificially from any of 

 these plants causes them within a short time to assume the 

 nocturnal position. Darwin suggested that the nocturnal 

 position prevents the loss of heat by radiation and consequent 

 injury from light frosts. But it is not by any means certain 

 that this is its real purpose. 



