234 PRIMARY ARRANGEMENT OF TISSUES. 



Common bundles rise for a certain distance in acropetal direction through the 

 stem, and then curve outwards at a node, to enter a leaf inserted at that point. 

 Their course in the stem is most clearly understood by following them from their 

 point of exit in a basipetal direction, that is, downwards. The description of their 

 course in this direction also corresponds best to the facts, inasmuch as at least 

 in most cases the development of the common bundles begins at the point of exit, 

 and proceeds on the one hand towards the leaf, and on the other downwards in 

 the stem. 



From the point of exit in the node, the common bundle passes downwards 

 through a number of internodes, and then affixes itself to, and unites with, another 

 bundle, usually a common bundle, which makes its exit lower down. The junction 

 is effected in most cases at or near a node. 



The common bundles accordingly represent within the stem the anatomically 

 demonstrable trace of the corresponding leaves : they are therefore called bundles 

 of the leaf-trace, and the whole number of those which belong to one leaf form the 

 trace, or internal trace of it ^- 



'-- The number of the bundles of a leaf-trace is constant, within narrow limits 

 of variation, for each region of the stem of each species : but differs greatly accord- 

 ing to the region and the species, varying from one to sometimes very high figures : 

 1 the leaf-trace thus generally consists of one or many bundles. 



A leaf-trace with many bundles may be distributed over a variable portion of 

 the transverse section, or circumference of the stem, or, as Nageli describes it, 

 it may be of variable width. The latter may amount e.g. to ^, \, \ of the 

 circumference of the stem. Traces with one bundle, or narrow ones with more 

 than one, become as a rule smaller, or narrower in a basipetal direction: wide 

 traces with more than one bundle usually increase in width in the same direction, 

 so that a lower one is enclosed by the one directly above it. 



The number of the internodes, which a bundle of the trace, or a whole trace 

 traverses before it reaches the point of junction, is constant within narrow limits 

 of variation for each definite individual case, according to species and region: 

 further, it is not less various in different individual cases than the above-named 

 relations. 



The individual bundle of a trace either remains undivide'd during its downward 

 course, or it may be split into two or more shanks. The bundles of a multiple 

 trace, as also of successive traces, may descend side by side, being thus concomitant; 

 or they are separated from one another by other bundles, which pass between 

 them, and pectinate '' with them. 



From the above it is plain that where bundles of a leaf-trace are present, 

 a definite relation exists between the arrangement of the leaves at the periphery, 

 and that of the bundles of the leaf-traces within the stem. If all bundles of leaf- 

 traces were separate and concomitant, and had a perpendicular course, their arrange- 

 ment in the transverse section of an internode would exactly correspond to the 

 horizontal projection of the arrangement of those leaves, whose traces pass through 



' Hanstein, /. c. a [Compare foot-note, p. 128.] 



