DICOTYLEDONS 



573 



summit, the foliar bundles originating from it later (in Hippuris, Aldrovanda, Cerato- 

 phyllum, and to a certain extent also Trapa, according to Sanio), it is the general rule 

 that 'common' bundles are first formed, the ascending branches of which enter the 

 stronger foliage-leaves generally in large numbers, and then pursue their course as isolated 

 bundles in the leaf-stalk and mid-rib, giving off the secondary bundles which constitute 

 the venation of the lamina^ The branches which descend into the stem mostly run 

 downwards through several internodes, become first interposed between the upper parts 

 of the older bundles, and sometimes (Fig. 434) first split and then coalesce laterally 

 with the older bundles lower down. Sometimes (as in Iberis) every bundle is twisted 

 in the stem and in the same direction, so that the bundles which have coalesced 

 sympodially, belonging to leaves of different heights on the stem, ascend spirally within 

 the bark. But most commonly they run parallel to the axis of the stem, until they 

 anastomose with older bundles lower down. The bundles do not bend deeply into the 

 inner tissue of the stem, but turn downwards and run parallel to one another at the 

 same distance below the surface, so that they lie in one layer, which presents the 

 appearance of a ring on transverse section separating the fundamental tissue into pith 

 and primary cortex. The portions of the fundamental tissue which lie between the 

 fibro-vascular buntilcs connect the pith with the primary cortex, and form the primary 



Fir,. 434. — The course of the bundles in two internodes of Satnbucus P.biihis: they lie in a cylinder wliicli is here 

 flattened out ; eaci) internode be.irs two opposite leaves, and each leaf receives from the stem a middle bundle h h and 

 two strong later.il bundles s' s' ; tlie descendint? arms of the bundles split and interpose between the lower bundles; 

 there arc in .i<lilition weaker bundles s" s' united by horizontal branches, from wliich bundles it « ascend into the 

 stipules. (After Hanstein.) 



Medullary Rays. If there is no subsequent increase in thickness no further change takes 

 place. But usually, even in annual stems (as Helianthus and Brassica) and invariably in 

 woody stems and branches several years old, the subsequent increase in thickness 

 begins after the elongation of the internodes. A layer of cambium is formed between 

 the outer phloem and the xylem which is turned towards the axis of each foliar bundle ; 

 the cambium layers of the bundles which are at first still separated by the medullary rays 

 lying side by side in a ring, unite in a closed mantle of cambium ; an interfascicular 

 cambium is formed by divisions in the intermediate cells of the medullary rays, and 

 bridges over the spaces between the separate layers of the cambium of the fibro- 

 vascular bundles (see Fig. 82, p. 95). The Cambium-ring thus formed produces on the 

 outside layers of phloem, on the inside layers of xylem, while it is at the same time 

 itself constantly increasing in diameter. All the tissue formed from the cambium-nng 

 on the outside may be termed Secondary Cortex, all the xylem formed on the inside 



» When several fibro-vascular bundles enter a leaf-stalk. Ihey are generally widely separated 

 by the fundamental tissue; but sometimes, as in the fig, the bundles aie arranged in a circle on 

 transverse section, and form a closed hollow cylinder which divides the fundamental tissue of the 

 leaf-slalk into pith and cortex. Isolated fibro-vascular bundles also run into the pith of the leaf-stalk 

 ill the fig, as occurs also in some stems of Dicotyledons. 



