48 



PLANT RELATIONS. 



small plants growing in exposed situations, as bare rocks 

 and sandy ground. The cluster of leaves, fiat upon the 

 ground, or nearly so, and more or less overlapping, is very 

 effectively arranged for resisting intense light or drought 

 or cold (see Mgs. 11, I'i, 4S). 



ijS. Protective positions. — In other cases, a position is 

 assumed hy the leaves -which directs their flat 

 surfaces so that they are not exposed to the 

 most intense rays of light. The so-called " com- 



Fto. 41. A leaf of a eensitive plant in two conditions. In the figure to the left the 

 leaf is fully expanded, with its four main divisions and numerous leaflets well 

 spread. In the figure to the right is shown the same leaf aftur it has heen 

 "shocked" by i sudden touch, or by sudden heat, or in some other way. The 

 leaflets have heen thrown tom'tber forward and upward ; the four main divisions 

 have heen moved together ; and the main leaf-stalk has been directed sharply 

 downward. The whole change has very much reduced the surface of exposure. — 

 After DucnARTRE. 



pass plants, ■ already mentioned, are illustrations of this, 

 the leaves standing edgewise and receiving on their surface 

 tlie less intense rays of light (see Figs. 5, 165). In the 

 dry regions of Australia the leaves on many of the forest 

 trees and shrubs have this cliaracteristic edgewise position, 

 known as the profile posi/ioii, giving to the foliage a very 

 curious appearance. 



Some leaves hav(^ the power of shifting their position 

 according to their nec^ds, directing their flat surfaces to- 

 ward the light, or nnn'e or less inclining them, according 



