72 COMPOUND ORGANS OF PLANTS. 



CHAPTER VI. 



THE LEAVES. . 



Leaves are contrivances by which the green absorbent sur- 

 face of the plant is increased, so that the greatest practicable 

 amount of food is taken from the air. The entire structure of 

 the leaf proves it to be put forth for this purpose. 



The leaf is simply an expansion of the green cellular bark 

 of the young shoot, and is formed by the spread of the woody 

 fibre which issues from its side, carrying with it at the same 

 time the bark, which thus becomes expanded horizontally, to 

 the air and light of heaven. 



When the leaf is fully developed it consists of two parts, 

 viz. : the expanded portion called the lamina or blade, and its 

 little stalk or support, which is termed the petiole. Some- 

 times, however, the petiole is wholly absent from the leaf, the 

 spread of- the woody fibre, together with the expansion of the 

 green young bark of the young shoot, taking place at its sur- 

 face. In this case the leaf is said to be sessile. So also this 

 expansion of bark does not always take place at a single point 

 of the stem, but is extended down the stem a little and then 

 spreads out horizontally, producing a decurrent leaf. The 

 leaves of the Verbascum thapsus, or common mullein, are of 

 this description. Occasionally, as in the orange, the bark of 

 the petiole itself shows this tendency to expansion, when the 

 petiole is said to be winged. Blost frequently, however, dis- 

 tinct fasciculi or bundles of woody fibre and spiral vessels 

 emerge from the side of the shoot, unite and form a petiole, 



