272 



PRTMARl' ARRANGEMENT OF TISSUES. 



down, near to the middle of the stem, and close to that of the next lower leaf, through 

 one internode, and then unites with the latter in the node (comp. Fig. 124), In a 

 transverse section of the internode there are accordingly two bundles of the leaf- 

 trace, which are close to the centre in the diameter betw-een the median lines of the 

 two rows of leaves. A small vertical cauline bundle appears at a later stage than the 

 leaf-traces and near to them, and this lies in the radial longitudinal plane at right 

 angles to the plane of the median lines of the leaves : in the nodes transverse and 

 oblique anastomoses appear at an early stage, as in P. natans. 



P. densus shows fundamentally the same structure, with the striking difference 

 that each bundle of the leaf curves almost at right angles into the middle of the stem, 

 and inserts itself in the next lower node directly on the bundle which there passes 



'^Vi^sr^ 



'^/^ftrS^ 



Fig. 122 (145). — Potajnogeton natans. Axile body of the internode, containing the vascular bundles : transverse 

 section, u endodennis thickened on one side (with starch) ; outside this the lacunar cortical parenchyma (with much 

 starch) ; / air cavities. Explanation of the figures in the text. The groups of delicate tissue of the numbered areas 

 are the phloem portions ; the wide meshes in them are the sieve-tubes of the bundles ; the areas in which the figures 

 stand are their vascular portions (for the most part converted into cavities). Between the bundles is starchy 

 parenchyma, and sclerenchj-matous fibres, with a narrow lumen, which appears as a dark point. 



out, SO that only one axile sympodium of the leaf-trace is present besides the 

 two cauline bundles. In the upright stems of P. pectinatus (Fig. 123), in P. pusillus 

 and Zanichellia palustris this axile sympodium is alone present, without the two 

 cauline bundles, to w-hich fact we shall return later. 



P. crispus shows a somewhat different arrangement, which will be described 

 below. 



It is not improbable that this type of vascular bundle-system is allied to that of 

 Hydrocharis, Stratiotes, and their allies, but further investigations are wanted on this 

 point. 



The transverse section of the stolons of Hydrocharis ]Morsus Ranae ' shows four bundles 



^ Rohrbach, Beitr. zur Kenntniss einiger Hydrocharideen, Abhandl. d. Naturf. Ges. z. Halle, 

 Bd. XII. p. 75. 



