i82 HISTORY OF BOTANY 



Tieghem is derived from the last named by the degenera- 

 tion of the internal phloeoterma, or endodermis. The 

 siphonosteUc type of central cylinder as defined above 

 is probably to be regarded as the result of the mechanical 

 strengthening of the cauUne axis to enable it to support 

 the pahngenetically large leaves which are characteristic 

 of the Angiospermae, Gymnospermae, and Fihcales. 

 In these three groups the siphonosteUc fibrovascular 

 cyUnder 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 cyhnder of the Lycopodiales and Equisetales, 

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

 gaps subtending the branches." 



All vascular plants thus possess a central cylinder 

 which is either protostelic or siphonosteUc. The protostele 

 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 cyUnder 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 

 FiUcales, prominent gaps are produced which, if the 

 leaves be closely set, may overlap so as to obUterate 

 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 cyUnder. 

 To these two conditions Jeffrey appUed the terms " phyllo- 

 siphonic " and " cladosiphonic " 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 Steldr-Theorie, and also by Tansley in his 

 Lectures on the Evolution of the Filicinean Vascular System, 

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



