THE LEAF 



201 



DIVISION 11. 



The Structure and Vital Processes of the Parts 



of Plants. 



I— THE LEAF 

 A. The Outer Structure. 



1. The Parts of a Leaf. — The chief parts of a leaf are: 



(a) the blade, which provides a large surface exposed to the 

 action of light and air, so as to enable the plant to evaporate its 

 water and to gather carbonic acid gas; 



(h) the stalk or petiole, which places the 

 blade in the most suitable position with re- 

 gard to the light (see Cucumber, page 75) 

 and protects it from damage by wind or rain 

 (see Mango, p. 25). 



Some leaves have no petioles, e. g., Ixora, 

 and are then called sessile. The petiole may 

 have appendages on it quite distinct from 

 the stem, as in the Orange (p. 19): it is 

 then called ivingecL If the blade of the leaf 

 runs down into the stem, as in Splmeranthus, 

 the leaf is termed decurrent. When the 

 petiole is not attached to the base of the leaf 

 but to its centre as in Castor or Taro (p. 147), 

 it is peltate. 



In many leaves (Horse Gram) the base Fig. i82. — Tipof a 

 of the petiole is thickened and flexible. Such ^I'^ss blade with parai- 

 a leaf is called pulvinate. 



(c) The sheath is an expansion of the 

 petiole surrounding the stem and protecting the tender parts 

 berieath it. It is common in Grasses and other monocotyledons. 



In the place of a sheath w^e find persistent or deciduous 

 appendages, the stipules, at the base of the leaf-stalk in some 



lel veins (4 times 

 enlarged). 



