312 THE FOLIAGE LEAF 



i.e. turned towards the upper face. In the larger veins 

 and in the midrib these fibre strands are frequently 

 massive, and they help, sometimes in very notable degree, 

 to. stiffen the mechanical structure of the leaf. The veins 

 of the leaf then act rather like the ribs of an umbrella. 



The rigidity of the leaf is partly maintained by the 

 turgor of the living cells and partly by the veins, and these 

 two factors vary in their relative importance in different 

 species. 1 Leaves in which the veins are sufficient by 

 themselves to keep the leaf rigid do not droop even 

 when their living cells are seriously depleted of water. 

 In other cases the base and centre of the leaf is kept rigid ' 

 by the larger veins, while the less supported tip and 

 edges are drooping. The edges and tip also suffer first 

 from loss of water by evaporation because they are 

 farthest from the source of supply. 



Other Forms of Foliage Leaf. While the great 

 majority of foliage leaves have the form of thin plates, 

 exposing a great surface in proportion to their bulk to 

 the light and air, there are many leaves which differ 

 from this type. Besides the succulent leaves (see p. 275), 

 which may be thick and flat, or oval or even circular in 

 cross-section, the commonest types are the needle-shaped 

 leaves (pine) and the long bristle-shaped leaves of many 

 grasses, especially those which grow in dry places. The 

 narrow leaves of other grasses often fold on the 

 midrib, the stomata being confined to the approximated 

 surfaces, or roll up, in dry weather. These characters 

 all tend to reduce or to protect the transpiring surfaces, 

 and are generally related to diminished water supply or 

 to diminished water-conducting capacity of the xylem. 



1 A thick and rigid cuticle and a thick-walled hypoderm may also 

 be important in maintaining the rigidity of leaves under severe water 

 loss. This is one reason why evergreen leaves of leathery texture such 

 as those of laurels do not readily droop. 



