212 The Anatomy of Plants BOOKII 



that he could distinguish in the meristem of the plerome 

 an outer part giving rise to the vascular bundles, and 

 having the limiting layer on each surface, and an inner part 

 from which the pith alone originated. The association of 

 the perimedullary zone with the bundles was seen in its 

 being visible on the inner side of each bundle when they 

 were separated by wide medullary rays but not being 

 traceable across the latter. It must correspond, therefore, 

 with pericycle and not with endodermis. Plot said, more- 

 over, that it forms the seat of the internal phloem groups 

 or strands characteristic of the Cucurbitaceae and other 

 Natural Orders. 



Towards the close of the century the stele theory was 

 minutely studied by Jeffrey in America. In the main, Van 

 Tieghem's results were confirmed by his investigations, but 

 several modifications were proposed on the basis of extended 

 observations. 



Jeffrey, who published his results in 1897 and the five 

 or six years which followed, carried his researches through- 

 out the main divisions of vascular plants, and was thereby 

 enabled to generalize. His objections to Van Tieghem's 

 position were stated by himself as follows : 



' The polystelic type of Van Tieghem is not characterized 

 by the repeated bifurcation of the epicotyledonary stele, 

 but there is primitively in the young stem of this type 

 a tubular concentric stele with foliar gaps subtending the 

 points of exit of the leaf-traces. The astelic type of Van 

 lieghem does not result from the separation of the con- 

 stituent epicotyledonary stele into its constituent bundles, 

 for in the young so-called astelic axis there are no bundles 

 present at all, but a collateral stelar tube with foliar gaps 

 subtending the leaf traces, through which the internal and 

 external phloeotermal sheaths communicate. The medul- 

 lated monostelic type of Van Tieghem does not originate, 

 as he states, by the dilatation of the epicotyledonary stele 

 and the formation of an intrastelar pith, for in favourable 

 cases the so-called medullated monostelic central cylinder 



