182 HISTORY OF BOTANY 



Tieghem is derived from the last named by the degenera- 

 tion 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 fibrovascular 

 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." 



All vascular plants thus possess a central cylinder 

 which is either protostelic or siphonostelic. The protest ele 

 has no medulla, but consists of a central xylem surrounded 

 by phloem and pericycle ; the siphonostele differs in 

 having a central medulla. On this cylinder are inserted 

 the vascular systems of the leaves and branches which 

 may create gaps in the cylinder wall. If the leaves be 

 small, as in the Lycopodiales, the insertion of their vascular 

 systems causes little or no disturbance, if large, as in the 

 Filicales, prominent gaps are produced which, if the 

 leaves be closely set, may overlap so as to obliterate 

 more or less the tubular character of the vascular axis. 

 Similarly, where the vascular systems of the lateral 

 branches join that of the main axis, ramal gaps are 

 formed, disturbing the continuity of the primary cylinder. 

 To these two conditions Jeffrey applied the terms " phyllo- 

 siphonic " and " dadosiphonic " respectively. 



The various interpretations of the anatomical structure 

 exhibited by different types of vascular plants were 

 summarised and criticised in 1902 by Schoute in his 

 monograph, Die Stelar-Theorie, and also by Tansley in his 

 Lectures on the Evolution of the Filicinean Vascular System, 

 published in 1907-8. To these works I must refer you for 



