CHAP, ii The Differentiation of the Tissues 213 



of the older stem may be seen to be derived from the 

 so-called astelic condition of the young axis by the degenera- 

 tion 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 nbro-vascular 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 Tieghem is derived 

 from the last-named by the degeneration of the internal 

 phloeoterma, or endodermis. 



The siphonostelic type of central cylinder as defined 

 above is probably to be regarded as the result of the 

 mechanical strengthening of the cauline axis to enable it 

 to support the palingenetically large leaves which are 

 characteristic of the Angiospermae, Gymnospermae, and 

 Filicales. In these three groups the siphonostelic fibro- 

 vascular cylinder is invariably distinguished by the presence 

 of gaps corresponding to the points of exit of the leaf- 

 traces, and in this feature offers a marked contrast to the 

 tubular central cylinder of the Lycopodiales and Equisetales, 

 in which there are no foliar lacunae, but, on the contrary, 

 gaps subtending the branches.' 1 



Jeffrey's own conclusions were that vascular plants show 

 two types of cauline central cylinder, protostelic and siphono- 

 stelic, of which the former, held by him to be the most 

 primitive, is concentric and without any central medulla. 

 The second type is tubular, the vascular tissue surrounding 

 a pith. The continuity of the tube is interrupted by 

 foliar and ramal gaps, or by the latter only. By the large 

 size of the foliar gaps and their consequent overlapping, 

 the tubular structure may cease to be very obvious in 

 the adult. It is primitively concentric, but in the Phanero- 



1 Jeffrey, 'Morphology of the Central Cylinder in the Angio- 

 sperms,' Trans. Canad. Inst., vol. vi (1900). 



