312 MORPHOLOGY OF ANGIOSPERMS 



for example, is characterized by foliar gaps such as are found 

 in the Filicales and Gymnosperms. Often in the seedling of 

 this genus there is present an internal limiting layer of the 

 stelar tissue which degenerates in the adult. Hence it may be 

 assumed, in the absence of negative evidence, that the pith of 

 Ranunculus belongs to the same morphological category as the 

 cortex. Marie, from a comparative study of the anatomy of 

 all the Ranunculaceae, has reached the conclusion that the genus. 

 Ranunculus is the starting-point from which all the other gen- 

 era of the order have been derived. It follows apparently that 

 the central cylinder of the Ranunculaceae in general is suscep- 

 tible of the same interpretation as that of Ranunculus. If the 

 central cylinder of the Ranunculaceae be siphonostelic with 

 foliar gaps, i. e., phyllosiphonic, it may fairly be assumed 

 that the central cylinder of Dicotyledons in general is to be 

 similarly interpreted, especially as foliar gaps are universally 

 present, even in such extreme cases of xerophytic reduction as 

 Casuarina and the Cactaceae. 



There are some instances of the occurrence of concentric 

 bundles in the Dicotyledons, but they appear to be of ceno- 

 genetic origin, and consequently of no phylogenetic importance ; 

 for in the cases which have been investigated, the concentric 

 condition is ordinarily absent in the seedling, the leaf-traces, 

 and the reproductive axes. This feature is illustrated by Pri- 

 mula farinosa, in which the bundles of the seedling, the repro- 

 ductive axis, and the leaves are always collateral ; whereas those 

 of the older vegetative stem are usually concentric. Similar 

 phenomena have been observed in the Xymphaeaceae, Halo- 

 raghidaceae, etc. 



In the older subterranean stem of Ranunculus acris the 

 fibrovascular tube becomes broken up into a series of segments 

 or bundles by the overlapping of the foliar gaps; quite often 

 in the stouter subterranean axis of Ranunculus acris (Jef- 

 frey 13 ) the foliar bundles tend to run in the pith before passing 

 out to the leaves, thus offering a striking feature of resemblance 

 to the normal course of the leaf-traces in the Monocotyledons. 

 In the aerial stem, however, this feature is not present, as may 

 be seen in Fig. 113, DD, in which the arrangement of the 

 bundles shown is the typical one for the Dicotyledons. There 

 *are a good many exceptions to the rule, however, e. g., Podo- 



