44 



COLLEGE BOTANY 



herbaceous stem skows four regions (Figs. 22 and 24) : the 

 epidermis, the cortex, the circle of fibro-vascular bundles and 

 the central or axis cylinder of pith. In the case of the very 

 woody stems we find the epidermis, the cortex, the fibro-vascular 

 bundles which are V-shaped and very compact, and a compara- 

 tively small axis cylinder (Fig. 27). 



The epidermis consists of a single layer of cells which forms 

 a covering over the surface of the stem. These cells are cutin- 

 ized and in young stems are very similar to the epidermal cells 



Fig. 22. — (a) CroBS-section of dicotyle- 

 donous stem; (&) fibro-vascular bundle 

 of same. 



Fig. 23. — (a) Cross-section of mono- 

 cotyledonous stem; (&) cross-section of 

 fibro-vascular bundle of same. 



of the leaf. They may vary somewhat in character, dependent 

 on the kind of plant or the climatic conditions under which the 

 plants are grown. The epidermis may be covered with trichomes 

 or plant hairs, which are, in reality, a part of it, and are of the 

 same general character as those found on the leaves. 



The cortex is composed of parenchyma or collenchyma cells 

 or both and lies just beneath the epidermis. These cells may 

 contain chlorophyll and perform the functions of foliage. In 

 old stems we frequently find elongated cells called sterome fibers 

 or sclerenchyma fibers. The inner layer is known as the starch 

 sheath and probably corresponds to the endodermis of the roots. 



