COURSE OF BUNDLES IN THE STEM. 235 



the internode \ This may be the case ; but in most instances the direct relation 

 between the two systems of arrangement is obscured and destroyed by oblique 

 courses, pectinations, splittings, and coalescences. 



The special phenomena of the distribution of bundles in the stem, which vary 

 greatly, subject to the above general rules, may in some few cases be recognised 

 as direct consequences of adaptation. For the most part they appear as anatomical 

 characters (p. 25) of the groups of various rank distinguished in systematic botany, 

 but vary individually within the groups of higher order, as freely as does the external 

 conformation of individual species : their differences are often closely correlated with 

 those of the latter, as might be expected a priori, but not uncommonly they show 

 unexpected deviations. 



In accordance with these facts as at present before us, the following synopsis of 

 individual phenomena will best be arranged according to the main systematic divisions, 

 while within these the arrangement will follow the phenomena of distribution of 

 bundles. How far more general rules for individual families and genera or for certain 

 categories of adaptation may be laid down within the main types first mentioned, will 

 be in part obvious from the description of individual cases ; while in other cases 

 decision on this matter must be withheld till further investigations have been made, 

 since in many families, especially of the Phanerogams, the course of the bundles 

 has been investigated in single examples only, or even not at all. 



I. Type of the Dicotyledons. 



Sect. 61, By this name is indicated that course of the bundles which is charac- 

 teristic of the stem of the very great majority of Dicotyledons: further, of those 

 Coniferce also which have been investigated, and of the Gnetacece with the exception 

 of Welwitschia. Among the Monocotyledons many Dioscoj-eae belong to this type, 

 and of Filicinese, the Equiseta and Osmundaceae ; these will however be treated 

 in the later sections dealing with these orders. 



All the primary bundles of this type are common bundles of the leaf-trace. 

 They curve inwards into the stem at the node, and from thence they take a radially- 

 perpendicular course down it, all of them remaining at about the same distance from 

 the middle and from the surface of the stem. Leaf-traces with one bundle always 

 pass down through more than one internode : this is also usually the case with those 

 having several bundles. The insertion of the bundles on such as emerge from 

 the stem lower down occurs as a rule at the nodes, or close to a node, and in such 

 a way. that they are connected in a unilateral-sympodial manner (Fig. 92), or in a 

 reticulate manner by means of shanks or diverging limbs, which are attached to the 

 neighbouring bundles on either side (e.g. Fig. 108). 



From this course of the bundles results the characteristic, general, primary 

 structure of the typical stem of the plants of this category. The bundles are 

 arranged in the transverse section in a broken, ring-like series, the ring or circle 

 of vascular bimdles. The remaining, chiefly parenchymatous tissue in which they 

 are imbedded, is separated into an axile, cylindrical or prismatic body, which fills 



' Compare Karbten, Veget. Org. d. Palinen. Abhandl. d. Berlin. Acad. 1847,"^. 208. 



