34 



NATURE OF THE EPIDERMIS 



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 

 begin 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. This 

 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 



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

 darkness. The same movements occur as a result of unfavorable temperatures 

 and humidity. 



these reactions and adjustments are purposive but not intelligent. 

 The movements are not called forth by consciousness but by 

 stimuli to which the irritable living substance is attuned or sensi- 

 tive. 



1 6. The Significance of Certain External Leaf Structures. — 

 Many features connected with the structure of the leaf furnish 

 admirable illustrations of the fitness of the leaf for the perform- 

 ance of its work. The stimulus of light and moisture have a 

 marked influence upon the external structure and form of leaves. 

 The epidermis is strikingly modified by such forces. In shade 

 plants the epidermis consists of a rather delicate layer of cells 

 with very thin cuticle. This gives sufficient protection to such 



