NATURE OF PLANTS 33 



must notice as the cells mature and become better protected that 

 their irritability changes and they respond in a different way 

 to light and gravity, etc. They begin to unfold and turn away 

 from their erect positions and at maturity assume a fixed posi- 

 tion to light, etc., that is quite different from the original posi- 

 tion. The position of the leaves of our pines and several other 

 evergreens is not controlled by light and as a consequence they 

 assume a variety of angles to the incident rays. 



The leaves of many plants do not have a fixed position but 

 show more or less motility during their entire life. Interesting 

 examples are seen in the sensitive plants and in members of 

 the bean and oxalis families where marked changes result in 

 the position of the leaves from alterations in the intensity of 

 light, temperature, or moisture. Many of these plants fold their 

 leaves when the temperature falls at sundown. Many flowers 

 close in the same way. These changes are popularly termed sleep 

 movements. They are caused not only by the changes in the 

 intensity of the light but also by changes in temperature and 

 moisture. In the case of these leaves the cells in the swollen 

 organ (the pulvinus) at the base of each leaf are kept full of 

 water so long as the temperature and external conditions exert a 

 suitable stimulus upon the plant, but when the conditions are 

 unfavorable certain of the cells on one side of the pulvinus lose 

 water and contract while the cells on the opposite side of the 

 pulvinus remain rigid; consequently, the leaves droop and fold 

 in various ways. This is due in part to the fact that the con- 

 tracted cells of the pulvinus are no longer capable of assisting in 

 the support of the leaf and also to the fact that the rigid cells con- 

 tinue to exert a pressure which bends the pulvinus towards the 

 side where the contracted cells are situated (Fig. 22). This fold- 

 ing of the leaves materially reduces the area exposed to the 

 atmosphere. Consequently there will be less loss of heat by 

 radiation when the temperature falls. This appears to be the 

 reason in the case of many plants for the folding of the leaves at 

 night. Transpiration will be reduced in the same way. Many of 

 our sensitive clovers (Meibomia, Lespedeza), sensitive peas {Cas- 

 sia), sensitive plant {Mimosa), are able to check the excessive' 

 4 



