VASCULAR SYSTEM OF STEM 57 



tance through which this independent course can be 

 traced is variable in this plant. Further, the lateral 

 fusions do not occur only at or near the nodes ; instances 

 may not unfrequently be found of fusions occurring at 

 various points in the internodes. 



That the arrangement and course of the vascular bundles in the 

 dicotyledonous stem are connected with the arrangement of the 

 leaves is an obvious fact. It may be seen in Helianthus, but is 

 more prominently shown in plants with regularly decussate 

 leaves (Cerastium, Clematis (Fig. 3), Stachys}. Still the arrange- 

 ment of the bundles may differ radically from that of the 

 leaves, and is to a certain extent independent of them. This 

 may be seen in such a case as that of Jberis amara, where the 

 bundles do not run longitudinally, but in tangential spirals 

 which have no direct relation to the arrangement of the leaves 

 (Naegeli) (Fig. 2). The arrangement of the bundles in the normal 

 dicotyledonous stem in a cylinder is due to the fact that each 

 bundle as it enters from the leaf passes towards the centre of the 

 stem for a certain distance only, which is approximately equal 

 for all ; each then curves gradually into a longitudinal direction. 

 As regards the bundle-arrangement, Helianthus is not a very 

 good type of an herbaceous Dicotyledon, still it illustrates the 

 most essential points, such as (1) the ring of vascular bundles 

 as seen in transverse section ; (2) the entry of the bundles of 

 the leaf-trace between the bundles connected with the higher 

 leaves ; (3) the lateral fusion of the several bundles at the node. 

 Since the fusions often* occur at points other than the nodes, 

 and since the independent course of the bundles of the leaf- 

 trace is of variable length, it cannot be regarded as a perfect type. 

 Other types are therefore recommended for investigation, in which 

 the vascular system has been carefully traced by Naegeli. In 

 most of these it may be seen how closely the arrangement of the 

 bundles is connected with (1) the arrangement of the leaves, and 

 (2) the number of bundles entering the stem from each leaf. 

 Iberis amara, leaves alternate, leaf-trace with 1 bundle (Fig. 2). 

 Lupinus, leaves alternate, leaf-trace, with 3 bundles. Cerastium 



