NATURE OF PLANTS 31 



ways. This is due in part to the fact that the contracted 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 continue to exert 

 a pressure which bends the pulvinus towards the side where the 

 contracted cells are situated (Fig. 22). This folding of the 



FIG. 22. Leaf position of the sensitive plant, Mimosa: A, in light. B, in 

 darkness. The same movements occur as a result of unfavorable tem- 

 peratures and humidity. 



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. Trans- 

 piration will be reduced in the same way. Many of our sensi- 

 tive clovers (Meibomia, Lespedeza}, sensitive peas (Cassia), 

 sensitive plant (Mimosa), are able to check the excessive loss of 

 water on hot dry days by the folding of their leaves. As soon 

 as the loss of water becomes detrimental some of the leaves be- 

 gin to fold and the reduction of leaf surface continues until the 

 loss of water by transpiration is met by root absorption. Very 

 commonly leaves that do not have this power of adjustment are 

 able to accomplish the same results by a rolling of the leaf. Thi? 

 is particularly noticeable among the grasses, which, on hot, dry 

 days, roll up their leaves so tightly as to change the appear- 

 ance of the plants. The interesting feature of all this is that 



