33] CHAPTER II. THE TISSUES. 123 



of this (as in Osmundacese, most Gymnosperms and Dicotyledons) 

 the bundles (leaf-traces) entering the stem from a leaf are few in 

 number, or even only one ; they penetrate to an egu^l depth in the 

 stem, 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 bifurcate so that they join 

 with the bundles of the lower leaves (Fig. 99 JB}. When the 

 leaf-traces entering the stem from a leaf are more numerous, they 

 penetrate to various depths in the stele, and tljeir course is 

 usually not vertical, but more or less curved : they may then form 

 two circles (e.g. Cucurbitacese, Phytolacca, Piperacese) ; or many 

 circles, more or less irregular, trenching on the pith (e.g. many 

 Ranunculacese, such as Cimicifuga, Thalictrum ; Nymphseacese ; 

 Monocotyledons generally). A good example of this is afforded by 

 a Palm stem (Fig. 99^4). The median leaf-traces first tend to- 

 ward the centre of the stem ; they then bend outward, thinning 

 out gradually as they descend, and coalesce with the lateral bun- 

 dles, which do not penetrate so deeply, in the pericycle at a point 

 much lower down. Furthermore, each bundle is somewhat twisted 

 in its course, so that the lower end lies toward a different side of 

 the stem from that on which it entered it. In these cases, when 

 there is a well-defined external ring, the more internal bundles 

 are termed medullary bundles. 



The relative position of the phloem and of the xylcm in a con- 

 joint bundle is subject to some variation ; they may either be 

 side by side, when the bundle is said to be collateral ; or the one 

 may more or less completely invest and surround the other, when 

 the bundle is said to be concentric. 



In the collateral bundle, the wood and the bast are so situated 

 that they both lie on a straight radial line drawn through the 

 bundle from the centre of the member to the surface, the wood 

 being nearer the centre, and the bast nearer the surface (see 

 Fig. 97). This type of bundle is common in the stems and leaf- 

 stalks of Phanerogams and of some Pteridophyta (Osmundacese, 

 Ophioglossacese, Equisetum). 



In some stems (e.g. Solanacese, most Convolvulacese, Cucurbitacese, 

 etc.) there is a second bast-bundle on the inner (medullary) side 

 of the wood of the conjoint bundle ; such a bundle is distinguished 

 as bicollatcral. 



In a concentric bundle, either the bast is surrounded by the 

 wood, or the wood by the bast, more or less completely : the 



