3i6 PRIMARy ARRANGEMENT OF TISSUES. 



In another series of cases the bundle of the root divides at the outer limit of the 

 stem-cylinder into numerous branches, which, diverging in all directions, enter 

 between the bundles of the stem, and penetrate with a sinuous course to the middle 

 of it, and then insert themselves, some of them further out, others further within, on 

 bundles of the stem. This is the case in the Palms \ where the penetrating bundles 

 do not reach the middle of the stem, in the nodes of the Grasses, and thick-stemmed 

 Commelinese. 



Also in the thick primary lateral roots of Pandanus the bundle-system of the 

 corresponding lateral axis inserts itself on that of the main axis in the manner just 

 described, i. e. numerous radiately diverging and undulating branches pass between 

 and up to the longitudinal bundles, which will be described below (Sect. io8) : also 

 in the Palms, according to Mohl, the bundle of the lateral roots splits in a similar 

 way into thin branches, and penetrates between the elements of the hollow cylindrical 

 bundle of that which is relatively the main root. 



The phenomena in the above Comraelineae should be rather more exactly described. 

 Several lateral roots arise at the nodes, especially on the side opposite to the axillary 

 shoot, and somewhat higher than its point of insertion, and the point of union of the 

 bundles of the leaf-trace. The bundles of the latter penetrate horizontally into the stem 

 as far as the cylindrical surface occupied by the ring of cauline bundles (see p. 269). , 

 Here they divide into horizontally diverging shanks, and these together form a slight 

 transverse girdle passing round the whole periphery. In the thin stems of Tradescantia 

 albiflora and Commelina agraria this girdle is without centripetal branches. In Tr. zebrina, 

 virginiana, and Maravelia zeiylanica numerous branches pass down from it in a centripetal 

 direction, they are distributed transversely through the whole node, with sinuous curva- 

 tures, and anastomose with the descending bundles of the stem, and with those which 

 enter from the axillary shoot, thus forming a felt, which is less dense and deep than that 

 which is characteristic of the nodes of Grasses, but is similar to it. 



B. STRUCTURE OF THE VASCULAR BUNDLE. 



Sect. 99. The vascular bundles are strands which consist of tracheae and 

 sieve-tubes as their essential parts. Both are accompanied by parenchymatous, and 

 often by sclerenchymatous elements. The structure of the bundle is determined by 

 the juxtaposition of all these component parts. 



A bundle undergoes in its course more or less considerable changes. Cross- 

 sections of the same bundle, taken at a distance from one another, may show the 

 greatest differences in the number and distribution of the individual elements; 

 Fig- 147; for example, represents the part of a bundle of the leaf-trace of Acorus Ca- 

 lamus passing through the leaf, while Fig. 148 shows its lower end as existing in the 

 stem ; in the intermediate tract the one structure passes gradually over into the other. 

 Such differences come out most conspicuously on comparing the peripheral ends of 

 the bundles, where they are spread out in the leaves or periphery of the stem, with 

 the other parts of them. It is therefore expedient in considering them to distinguish 



' Von Mohl, Palm. Struct, p. xix. Tab. I. A, and Verm. Schr. pp. 157 and 172. 



