198 



PART II. VEGETABLE HISTOLOGY. 



color of the upper surface, owing to the compact arrangement of 

 the chlorophyll-bearing parenchyma cells next to it, is usually a 

 much deeper green than that of the lower surface. 



The centric arrangement does not ordinarily occur, save in 

 leaves that are considerably thickened, as in those of Aloes and 

 Crassula, and in leaves which, like those of the Pines, do not pre- 

 sent a distinct upper and under surface. 



In all leaves, the stomata are placed directly over an inter- 

 cellular space, so that there is free communication between the 

 interior of the leaf and the outside. Fig. 458 represents a portion 



b 



Fig. 459. — Transverse section of a centric leaf, that of a Pine, a, epidermal cells, 

 excessively thickened; b b, stomata; c, thick -walled hypodermal cell; d, chlorophyll-bear- 

 ing parenchyma cells with walls much folded internally; e, endodermis; f, secretion reser- 

 voir; g, pitted parenchyma cells: h, xylem of a fibro-vascular bundle. Magnified about 50 

 diameters. 



of the cross-section of a bi-facial leaf, that of Cycas revoluta, 

 and Fig. 459 represents the cross-section of a centric leaf, that 

 of a species of Pine. 



As regards the Histology of the Floral Organs, it cor- 

 responds in a general way with that of the organs of vegetation 

 from which they are derived. They are, however, morpholog- 

 ically degenerate structures, and hence, on the whole, have less 

 complex tissues. Floral leaves, unless they contain chlorophyll, 

 develop few stomata or frequently none at all, and the epidermis 

 generally is a less complicated structure than that of ordinary 

 leaves. Its cells often contain a diffused liquid coloring matter 



