172 



PART II. THE INTIMATE STRUCTURE OF PLANTS. 



[33. 



traces do not, however, follow a uniform course in all cases. 

 Thus, they may proceed to the centre of the stem and form a 

 solid vascular cylinder (e.g. Isoetes among Pteridophyta ; and 

 certain aquatic Monocotyledons ; species of Potamogeton, Zanic- 

 hellia, Althenia, Ruppia). Or they may form a hollow cylinder. 



In the simplest case 

 of this (as in Osmun- 

 daceae, most Gymno- 

 sperms and Dicotyle- 

 dons) the bundles 

 (leaf-traces) entering 

 the stem from a leaf 

 are few in number, or 

 even only one ; they 

 penetrate to an equal 

 depth in the stem 

 (also in the schizo- 

 stelic Equisetum, 

 Fig. 116), and run 

 vertically downwards 

 through one or two 

 internodes, joining at 

 a node with the 

 bundles entering the 

 stem from a lower 

 leaf ; sometimes their 

 lower ends are bifur- 

 cate so that they join 

 with the bundles of 

 the lower leaves (Fig. 

 132 B). When the 

 leaf -traces entering 

 the stem from a leaf 

 are more numerous, 

 they penetrate to 

 various depths in the 

 stele, and their course 

 is usually not vertical, 

 but more or less 

 curved : they may then form two circles (e.g. Cucurbitaceae, 

 Phytolacca, Piperaceae) ; or many circles, more or less irregular, 



FIG. 132. Diagram of tbe course of the vascular 

 bundles in stems. A Longitudinal section through the 

 axis of a Palm-stem, showing a transverse section of half 

 of it. The leaves (cut off above the insertion) are hypothe- 

 tically conceived of as distichous and amplexicaul, and so 

 are seen on both sides of the stem, 1 m 2 m3 m being the 

 median line of each. B Outside view and transverse 

 section of Cerastium (hypothetically transparent, to show 

 the internal bundles). The decussate leaves (1, 2, 3) are cut 

 off. The bundle proceeding from ench leaf divides into two 

 above the leaf immediately below it, and the branches of 

 all the bundles unite to form the four thin bundles which 

 alternate in the section with the thicker ones. In the 

 section, m is the pith, r the cortex, v the medullary ray. 

 The xylem in the nbro-vascular bundles is indicated by 

 shading. 



