SECT. I 



MORPHOLOGY 



129 



for mechanical purposes, into strands of collenchyma or sclerenchyma. Such a 

 mechanical tissue, which" serves to strengthen the epidermis, is known as a 





Fig. 138.— -4, Part of transverse section of a young stem of Aristolochia Sipho. e, Epidermis ; pr, 

 primary cortex ; st, starcli-slieath ; c, central cylinder ; pc, pericycle, in this case ^\itha ring of 

 sclerencliyma fibres ; cv', phloem, and cv", xylem portions of the vascular bundle ; cb, cambiiun 

 ring; m, medulla; ms, primary medullary ray. (x 48.) B, Small portion of the periphery 

 of a similar section of a still younger stem, e, Epidermis ; pr, primary cortex ; st, starcli- 

 sheath with easily movable starch grains ; pc, outer layers of the pericycle. (x 350.) 



HYPODERMA. Of the tissues composing the central cylinder, the pericycle, the 

 primary medullary rays, and medulla consist of fundamental tissue, and are 

 chiefly composed of colourless 

 parenchyma. A part, however, 

 of the tissue of the pericycle may 

 become sclereuchymatous (Fig. 

 138 A, 2k) ; sclerenchymatous ele- 

 ments also often surround in- 

 dividual bundles as sheaths, or 

 accompany the phloem portion in 

 the form of strands (Figs. 131, 133). 

 Whenever such a sheath of scler- 

 enchyma is developed about a 

 bundle, it is interrupted on both 

 sides of the bundle, at the junction 

 of the xylem and phloem portions, 

 by parenchymatous cells, or by 

 cells which are only slightly 

 thickened and lignified. The ex- 

 , istence of these unthickened places 

 facilitates the exchange of water 

 and food material between the 

 vascular bundles and the funda- 

 mental tissue. AVhcn a common starch-sheath is not present in a stem, starch- 

 sheaths are sometimes found sur-ounding the individual vascular bundles, or 



K 



Fig. 139. — Transverse section of an adventitious root of 

 Allium Cepa. ep, Remains of the epidermis; ex, exo- 

 dermis ; c, primary cortex ; e, endodermis ; cc, central 

 cylinder, (x 45.) 



