STRUCTURE OF THE SHOOT 



67 



spaces between them filled with air (Fig. 29, 4 sp). These 

 cells contain the small rounded green bodies to which the 

 colour of the leaf is due. The bodies are known as chloro- 

 phyll corpuscles or chloroplasts, and the green colouring 

 matter as chlorophyll (Gr. chloros= green, pIiyllon = a. leaf). 

 It will be noticed that each stoma on the under surface 

 always opens into one of these air-chambers (Fig. 29, a). 

 Between the two layers runs the meshwork of veins (Fig. 30) . 

 We can understand from such a section why the lower skin 

 should be more easily removed than the upper. 



Structure of the stem. Turning now to the tissues of a 

 stem like the Buttercup (Fig. 31), Dead- 

 nettle (Fig. 34), or Bean, we find these 

 similar to and continuous in structure with 

 those of the root and leaf ; and they are 

 also similar in function. A layer of cells 

 the epidermis (Fig. 31, e) covers the outer 

 surface, the exposed walls of which are 

 thickened with a protective layer the 

 cuticle. Stomata occur here and there as 

 in the epidermis of a leaf. Beneath the 

 epidermis is the cortex (co), followed by a 

 ring of veins, or vascular bundles (v.b). In 

 the centre is the pith (p), which in older 

 plants breaks down, making the stem hollow. Broad rays 

 of tissue pass between the bundles from pith to cortex. 

 These are the medullary rays. These tissues epidermis, 

 cortex, vascular bundles, rays, and pith occur generally 

 in plant stems, but become greatly modified according 

 to the requirements of the plant. 



The epidermis may develop a very thick cuticle in ever- 

 greens, and is usually waxy and more or less impervious. 

 In water-plants the cuticle is often very thin or absent. 

 In some plants, hairs are so abundant as to produce a woolly 

 covering. Not uncommonly some hairs pour out a sticky 



Fig. 30. 



Skeleton of 



Box Leaf. 



k 2 



