THE STELE THEORY 181 



Van Tieghem received strong support from Strasburger, 

 who, however, suggested the term " schizostele " to 

 indicate the branches that enter the petioles, and also the 

 segments of the " monostele " in Van Tieghem's " astelic " 

 condition. 



Some years later the American botanist, Jeffrey, 

 brought forward various criticisms of Van Tieghem's 

 views and also formulated a modification of his own. 

 In his Morphology of the Central Cylinder in the Anglo- 

 sperms, published in 1900, he writes : " The polystelic 

 type of Van Tieghem is not characterised 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 Tieghem 

 does not result from the separation of the constituent 

 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 

 medullated monostelic type of Van Tieghem does not 

 originate, as he states, by dilatation of the epicotyledonary 

 stele and the formation of an intrastelar pith, for in 

 favourable cases the so-called medullated monostelic 

 central cylinder of the older stem may be seen to be 

 derived from the so-called astelic condition of the young 

 axis by the degeneration of the internal phloeoterma. 

 Van Tieghem's three types of central cylinder indicated 

 above are all modifications of a single type, which has 

 been designated by the writer ' siphonostelic.' In this 

 type the central cylinder is primitively a fibrovascular 

 tube with foliar lacunae opposite the point of exit of 

 the leaf traces. In the so-called polystelic modification 

 the central cylinder has internal as well as external 

 phloem ; in the astelic type of axis the internal phloem 

 is absent. The medullated monostelic type of Van 



