118 



BOTANY 



I'AUT 1 



the endodermis appear as dark spots (Fig. 130), but in a tangential 

 section as a wavy band. The CENTRAL CYLINDER of the stem contains 

 vascular bundles (cv), which, in the Equisetaceae, the Gymnosperms 

 and Dicotyledons (Fig. 128 A\ are arranged in a circle, whereas in 

 Monocotyledons (Fig. 127) they are irregularly distributed. In all 

 these cases the xylem portion of the vascular bundle is directed 

 towards the centre, and the phloem portion away from the centre of 

 the stem. That part of the peripheral tissue of the central cylinder 

 lying outside of the bundles is called the PERICYCLE (pc). If the 



B 



FIG. 128. A, Part of a transverse section of a young stein of Arittnliu-hiu xifiho. e, Epidermis ; jv, 

 primary cortex ; st, starch-sheath ; c, central cylinder ; pc, pericycle, in this case with a ring'of 

 sclerenchyma fibres ; ctf, phloem, and cv", xylem portions of the vascular bundle ; cb, cambium 

 ring; m, medulla; ins, 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, starch- 

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



bundles are arranged in a circle (Fig. 128 A\ that part of the 

 central cylinder enclosed by them is the PITH or MEDULLA (m), and 

 the tissue between the different bundles the PRIMARY MEDULLARY 

 RAYS. In the case of scattered bundles (Fig. 127), a distinction 

 between medulla and medullary rays is no longer possible. Wherever 

 there is no sharp distinction between primary cortex and central 

 cylinder, comparative investigation alone can determine whether a 

 tissue belongs to the primary cortex or to the central cylinder. 



Although the fundamental tissue of the primary cortex is mainly a 

 chlorophyll-containing tissue, portions bordering on the epidermis frequently become 

 converted, for mechanical purposes, into strands of collenchyma or sclerenchyma. 

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

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



