AXILE STRANDS 375 



arrangement of the conducting system to a very marked extent ; thus, 

 the course of the vascular bundles in the stem is in general closely 

 related to the scheme of phyllotaxis, whereas the leaf-arrangement affects 

 the disposition of the stereome only in a very minor degree. In short, 

 correlation between different organs or tissues exerts a far-reachinir 

 influence upon the arrangement of the bundles. 



A. COURSE OF THE VASCULAR BUNDLES IN LEAFY STEMS. 



In stems the vascular bundles generally follow a longitudinal 

 course ; they may run parallel to the longitudinal axis or else diverge 

 obliquely either in a radial or in a tangential plane. Considerable 

 variety prevails as to the manner in which bundles terminate above 

 and below. A bundle which passes out above into a leaf, is termed a 

 common bundle, the lower cauline portion being distinguished as the 

 leaf-trace. Cauline bundles lie wholly within the stem ; frequently 

 they have leaf-traces inserted upon them at various points. At its 

 lower extremity a bundle may either remain isolated (" separate " or 

 " individual " bundles), or it may fuse with another bundle (" united " 

 bundles). Sometimes each leaf sends but a single vascular strand into 

 the stem. If the leaf-trace comprises several bundles, these often 

 pectinate with strands belonging to other leaves ; the whole vascular 

 arrangement may be further complicated owing to the fact that some 

 or all of the bundles fuse with one another at certain points. 



After these preliminary remarks, we may pass on to describe the 

 principal types of arrangement of vascular bundles. 



1. The simplest and most primitive form of vascular system 

 consists of an axile strand, from which single bundles pass out into the 

 leaves. This axile strand may be cauline, or it may represent a 

 sympodial structure, built up of the cauline portions of successive 

 leaf- traces. 



In the stems of Polytrichaceae, in the adult stem of Hymcno- 

 phyllum, Gleichenia and Lygodium, as well as in " seedling " Ferns 

 generally, and also in certain species of Selaginella, this axile strand 

 constitutes a primitive central cylinder [protostele 179a ]. In the Polytri- 

 chaceae the leaf-traces join the concentric cauline strand, after traversing 

 the cortex in an obliquely radial direction ; those elements of the leaf- 

 trace which correspond to sieve-tubes and conducting parenchyma, become 

 merged in the leptome-sheath, while the tracheal tissue is continued 

 across the sheath in the original direction of the leaf-trace, finally 

 becoming inserted upon the central core of hadrome. In Mosses which 

 have a simple central conducting strand in the stem, the leaf-traces may 

 likewise be attached to this main trunk of the vascular system ; in other 

 cases, however, they do not penetrate so deeply into the stem, but end 



