THE PLANT-BODY 



25 



primary function of the leaf is the very important one of assimilat- 

 ing carbon-dioxide, and to facilitate 

 this, the green cells are spread out 

 in such a way as to offer the most 

 favorable exposure of the cells to the 

 action of light. The typical leaf 

 (Fig. 16) has a broadly expanded 

 thin lamina, or Blade, exposing a 

 maximum surface of green tissue to 

 the light. The vascular bundles 

 form a skeleton which gives the 

 necessary support to the leaf, and 

 at the same time they serve as chan- 

 nels for the conduction of water and 

 food. Covering the delicate green 

 tissue, and protecting it against loss 

 of water, is the epidermis, which is, 

 however, perforated by the stomata, 

 pores which permit communication 

 between the air-spaces within the 

 plant and the outside atmosphere. 



The leaf is usually connected to 

 the stem by a stalk or Petiole, which 

 is more or less modified, at the place 

 of junction, into the " Leaf-base," 

 from which there are often developed 

 leaflike appendages, or Stipules (Fig. 16). In case no petiole is 

 developed, the leaf is " Sessile," and occasionally two opposite sessile 

 leaves are coherent, as in the " Perf oliate " -connate leaves of some 



A B C 



st 



FIG. 16. Typical foliage leaf of 

 Pelargonium sp.; I, lamina; 

 p, petiole ; b, leaf-base ; st, stipules. 



FIG. 17. A, simple sessile leaf of Pogonia ophioglossoides. B, lobed leaf of Quercus 

 lobata. C, pinnately compound leaf of Rose; st, stipules. 



