STRUCTURE OF THE SHOOT 65 



spread out to the best advantage as regards light and air. 

 It is obvious, too, that the stem is the means of communica- 

 tion between root and leaf. In order to bear the weight of 

 leaves and branches, and to withstand the strain of heavy 

 winds, it needs to be strong ; and to resist the attacks of 

 numerous enemies, its outer surface must be tough or 

 otherwise resistant. To prevent the escape of sap, which 

 passes along the stem, it must be impervious. Structures 

 thus exposed to many and varied conditions, and having 

 to serve so many purposes, are likely to show a wide range 

 in duration and modification of form and structure. 



Though the leaf and stem differ in many details from the 

 root, they are built up of the same general tissues an 

 epidermis on the outside enclosing a cortex, and, within, 

 a ring of vascular bundles surrounding the central pith. 

 In a leaf, however, the blade of which is usually in the form 

 of a thin plate, the veins spread out in the form of a flat 

 meshwork. 



Structure of the Box leaf. A simple dissection will show 

 the relationship of these different, tissues of which the leaf 

 is composed. Boil a few Box leaves in a solution of caustic 

 potash for fifteen to twenty minutes, then wash gently in 

 water, and place them on a glass slip. Dissect off carefully 

 first the lower skin, then the upper skin, and mount them 

 on separate slips (Fig. 29, 1 and 2). There now remains the 

 skeleton, the meshes of which are covered by and filled in 

 with a soft green tissue. With a camel-hair brush carefully 

 remove this tissue, and so prepare a clean skeleton (Fig. 30). 

 Examine all these parts carefully with a pocket lens. How 

 do these skins differ ? Is one more readily removed than the 

 other ? and if so, which ? What structures do you find on the 

 under skin which are absent from the upper one ? Examine 

 these with a microscope ; and also the green tissue you have 

 removed from the meshes of the veins. Each of the dots, 

 seen with a lens on the under skin consists of two sausage-- 



1200. 5. 



