114 CONNECTION BETWEEN LEAF AND STEM 



opening towards the concave or flattened upper surface and with 

 the protoxylems directed inwards (Fig. 54, A-D). The part of 

 the ground-tissue included within the group of strands is spoken 

 of as pith, and that outside as cortex, the peripheral layers of 

 the latter not uncommonly consisting of 

 mechanical tissue (Fig. 54, Sc.). 



The bundles of the petiole can be traced 

 backwards some little distance into the 

 cortex of the stem, following a slightly 

 oblique course, so that, in sections cut 

 transversely just below the nodes, the one 

 or more bundles (leaf-trace bundles) passing 

 from the leaf into the stem appear cut 

 obliquely in the cortex. After penetrating 

 some little way into the latter the bundles 

 turn abruptly downwards and run vertically 

 through one or more internodes, ultimately, 

 with (Fig. 55) or without previous branch- 

 ing, fusing laterally with strands derived 

 from other leaves. The bundles traversing 

 the stem are therefore merely downward 

 continuations of those found in the leaves, 

 i.e. the bundles of stem and leaf are common. 

 The vascular strands passing to the axillary 

 buds (cf. Fig. 69, p. 142) are branches of 

 those serving the leaves, whilst the vascular 

 supply of stipules originates in a similar 

 manner. 



In Monocotyledons, where the sheathing 

 leaf-bases completely encircle the axis, 

 numerous strands pass from each leaf into 

 the stem. The median vascular bundles 

 pass almost to the centre of the stem 

 before they bend downwards ; subsequently they pursue a 

 longitudinal direction, at the same time returning obliquely 

 towards the periphery. The more laterally placed strands 

 follow a similar course, but do not penetrate to the same 

 depth. As a result the vascular bundles from the successive 

 leaves usually appear irregularly scattered through the cross- 



FIG. 5.5. Diagram- 

 matic representa- 

 tion of the vas- 

 cular system in a 

 small portion of 

 the stem of the 

 Mouse-ear Chick- 

 weed (Cerastium) 

 (modified from 

 Prantl). Only the 

 bases of the 

 leaves, with a 

 single vascular 

 strand, are shown. 



