STEUCTUEE AND WOEK OF PLANTS 



13 



furniture in the room to see if you can recognize the partial 

 rings of wood or can tell the way in which the timber was 

 sawed. In later chapters there will be a more extensive study 

 of stems and the ways in which they grow. 



14. Leaves, general form : the epidermis. Most leaves con- 

 sist of two parts, the leafstalk or the petiole, and the blade, 

 which is the expanded portion. In some leaves the petiole 

 is absent, and in others the blade is subdivided into several 

 parts, in which case the leaf is said to be compound. To most 

 observers leaves appear to be a uniformly green mass of mate- 

 rial. More careful observation discloses the fact that many 

 leaves are not .equally green on both surfaces, and that 

 running throughout the leaf there are more or less regularly 

 arranged veins or fibrovas- 

 cular bundles which are not 

 green. 



From the upper and lower 

 surfaces of leaves such as 

 those of live-forever, Wan- 

 dering Jew, Easter lily, and 

 epiderwort one may peel a 

 thin, almost colorless layer, 

 which is known as the epider- 

 mis (Fig. 10). The epidermis 

 is composed of cells more or 

 less compactly arranged. In 

 the epidermis from one and 

 sometimes from both sur- 

 faces there are special struc- 

 tures known as stomata 

 (Fig. 10). From a surface 

 view a stoma (plural, stomata) 

 presents two more or less crescentic or kidney-shaped cells, 

 the guard cells, between which is an elliptical opening, the 

 stomatal opening. Unlike other epidermal cells, the guard cells 

 are greenish. The opening serves as a place of entrance for 



FIG. 10. A surface view of leaf epider- 

 mis from the geranium (Pelargonium) 



Among the ordinary epidermal cells (c) are 

 four stomata, each with two guard cells (gc) 

 and the mouth of an air cavity (p). Con- 

 siderably magnified 



