194 



PART II. — VEGETABLE HISTOLOGY. 



or "filling" of the leaf; and (3) the epidermis, which covers the 

 whole. 



The epidermis of the leaf has substantially the same structure 

 as that of the stem, except that it is usually more abundantly 

 supplied with stomata, particularly on its under surface. Here 

 they are often very numerous. The leaf of Osmunda regalis 

 has 21,000 to the square inch, that of the Apple upwards of 

 150,000, and that of the Olive tree more than 400,000. The 

 ordinary epidermal cells are also more apt to have lobed or 

 irregular forms than those of the stem, as illustrated in Fig. 405. 



The fibro-vascular bundles of the leaf are 

 usually composed of much the same elements 

 as those of the stem with which they are contin- 

 uous, except that in their finer ramifications 

 they become much depauperated, being often 

 reduced to scarcely more than a row of tra- 

 cheids. They follow the course of the veins 

 and are a constituent part of them, though the 

 veins include other tissues than those which 

 properly belong to the fibro-vascular system. 

 The bundles are usually collateral and closed ; 

 the phloem portion faces the lower or outer 

 surface, while the xylem faces the upper or 

 inner surface. In the petiole there is either 

 one large bundle or, more commonly, a mass °, f °. smu,lda regah 



* showing arrangement 



composed of several pursuing a parallel course, of fibro-vascular sys- 



1 . . tem - Magnified two 



at the stem end, becoming a part of its fibro- diameters, 

 vascular system, and in the lamina spreading out in various 

 ways, according to the plans of venation already described. 



In the forked type, seldom seen except in Ferns and their 

 allies, the bundles may spread out from the base of the leaf and 

 run toward the margin, in which they terminate without anasto- 

 moses, or they may diverge from a median group of bundles or 

 midrib, as shown in Fig. 454. 



The parallel type is, as we have already seen, the prevalent 

 one in Monocotyledons and Gymnosperms, though in exceptional 

 instances it occurs among Dicotyledons, as in Eryngium yuccse- 

 folium. 



It presents interesting variations. In the Pines a single 



Fig. 454. — Leaflet 



