48 PRACTICAL BOTANY. 



leaves, as far as they can be traced independently and 

 without fusion. This will be possible at least for one 

 internode, and usually for two or three; but the 

 distance 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 $f 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 

 (cf. Cerastium, Clematis, Stachys}. Still the arrangement 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 Iberis amara, where the bundles do not run longi- 

 tudinally, but in tangential spirals which have no direct relation 

 to the arrangement of the leaves (Nsegeli). 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 ; e.g., (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. 

 We therefore recommend a series of types for investigation, in 

 which the vascular system has been carefully traced by Nsegeli. 

 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. Lnpimis. 



