FOLIAGE LEAVES. 



399 



the members of the family Leguminoseae (clovers, peas, beans, 

 etc.), and by the sensitive plants.* But it is also shared by 

 some other plants as well (oxalis, for example). The leaves 

 of these plants are usually provided with a mechanism which 

 enables them to execute these movements with ease. There is 

 a cushion (pulvinus) of tissue at the base of the petiole, and in 

 the case of compound leaves, at the base of the pinnae and pin- 

 nules which undergoes changes in turgor in its cells. The col- 

 lapsing of the cells by loss of water into the intercellular spaces 

 causes the leaf to droop. When the cells regain their turgor 

 by the absorption of the water from the intercellular spaces th<. 

 leaf is raised to the horizontal, or day position. The light stimu 

 ulus induces turgor of the pulvinus, the disappearance of the stim- 



Fig. 438. 

 Sunflower with young head turned toward morning sun. 



ulus is accompanied by a loss of turgor. It is a remarkable 

 fact that in some sensitive plants, intense light stimuli are alarm 

 signals which result in the same movement as if the light stim- 



* The most remarkable case is that of the "telegraph" plant (Des- 

 modium gyrans). Aside from the day and night positions which the 

 leaves assume, there is a pair of small lateral leaflets to each leaf which con- 

 stantly execute a jerky motion, and swing arovnd in a circle like the second 

 hand of a watch. 



