710 TRANSACTIONS OF SECTION K. 



same plant. It seems to emerge from such comparisons that, as regards the 

 xylem at least, a central strand may be independent of influences from the 

 leaves, while the latter may not only determine the leaf -traces connecting with 

 the central strand, but may influence the periphery of this ; the result is a 

 cylinder of outer xylem continuous with the leaf-traces. This general conception 

 is borne out by widely different plants, the correspondences between which are 

 homologies of organisation. I may instance the stele of the Polytrichaceae as 

 analysed by Mr. and Mrs. Tansley, the stele of the rhizome and aerial shoots of 

 the Psilotacese, of the Lycopods with larger or smaller leaves, and the stele 

 of the ferns at various ages of the plant. The shoot of Isoetes, which is of the 

 Lycopod type but has relatively large leaves, shows the composite nature 

 of the stelar xylem particularly clearly and also suggests how the component 

 influences are at work in the meristematic region of the stem bringing about the 

 resultant structure." 



Owing to the small size of the shoot-apex it is difficult to induce deviations 

 from the normal to show the respective parts played by the central axis and by 

 the influences from the leaf-primordia. The reality of influences proceeding 

 backwards from developing structures is better brought out when they may be 

 present or absent, and for this lateral buds are of special interest. As a rule, 

 the primary development of buds has proceeded far enough to determine the 

 connecting vascular tracts, but in the case of the dormant axillary apices of 

 Botrychium no influence has been exercised on the vascular structure of the 

 main shoot. When, however, such a lateral apex is called into activity, it not 

 only forms its own vascular system as it develops, but exerts an influence back- 

 wards through permanent tissue leading to the production of a ' branch-trace ' 

 connecting with the adaxial face of the subt-ending leaf-trace. In Heliain- 

 thostacliys a similar connection is established with the stele of the main stem, 

 and the influence may extend to the whole periphery of the main stele, inducing 

 a continued or secondary production of xylem both behind and before the place 

 of insertion of the branch. 



These constructions were in a sense called fortli by experimental interference, 

 since they occurred in plants the normal apical growth of which had been 

 arrested. Plants of Osmunda are normally unbranched and no indication of 

 dormant lateral apices have ever been detected. I tried on young plants of 

 Osmunda regalis the experiment of injuring or destroying the apex of the shoot, 

 with the result that in a number of them branching was induced. The vascular 

 relations exhibited considerable variety, but in some clear cases the branch was 

 developed in an axillary position with regard to a leaf -primordium "= and its 

 vascular connection was with the adaxial face of the subtending trace in the 

 same fashion as in Botrychium and in some species of Zygoptcris. The disturb- 

 ance of the normal growth had apparently brought out (in more or less irregular 

 form) the system of relations governing the position of development of lateral 

 brandies. The result showed the correspondence with what is the normal con- 

 dition in some Zygopterideae. It has been said from the phyletic side, and on 

 the whole rightly, that experiment cannot reconstruct history. In the light of 

 Dr. Kidston and Professor Gwynne-Vaughan's conclusions as to the derivation 

 of the Osmundaceae from a Zygopterid ancestry this induced branching of 

 Osmunda might almost be cited as a partial exception to the statement. 

 _ These examples will suffice to indicate the justification for a change of attitude 

 in the study of the vascular system. Looked at in this light, the stele appears 

 not as a characteristic thing inherited as such, but as a complex resultant. Tlie 

 problem gams in interest, new questions (which are different from, though not 

 antagonistic to, jihyletic problems) can be put as to stelar structure, leaf-trace 

 structure, the venation of leaves, &c. We see this if we glance at the pro- 

 gression m stelar structure that accompanies the development of the young fern. 



"_ This is confirmed by the origin of a similarly differentiated ' stele ' in 

 relation to the insertion of the successively produced roots at the base of the 

 plant of Isoetes. 



'^ In another case the leaf primordium appeared to be replaced by an axillary 

 branch— c/. the case of Plagiogyria described by Bower, Annals of Botany, 

 vol. xxiv. p. 434. ' " 



