PRESIDENTIAL ADDRESS. 711 



The pliyletic explanation has been recapitulation. We have found reason to 

 criticise the adequacy of this as applied to external form, and the same 

 line of criticism applies to the stelar progression. In this also the early staoes 

 may be hurried over or absent and, still more significant, the early type of 

 stelar structure may recur, w^hen the shoot has fallen upon evil days and 

 approximated in size of stem and leaf-form to the seedling condition. From 

 such points of view the vascular system offers problems in general or causal 

 morphology not merely of great interest but with some possibility of solution. 

 Thus the parallel progressions from protostely to a medullated monostele, and 

 from protostely to solenostely and dictyostely may be treated as problems in 

 the expansion and condensation of a stelar structure, which is itself the re- 

 sultant of a system of influences. Such parallel progressions are before us within 

 the ferns and also in other groups of Vascular Cryptogams. 



One of the most remarkable of these (which also affords an example of a 

 change in anatomical construction related to a change in the external conditions) 

 is seen in the occurrence of a solenostele in the rhizome of Selaginella Lyalli. 

 This has been explained by the relation of the aerial branch-systems to the 

 rhizome being similar to that of the leaf to the creeping fern-stem. Professor 

 Bower's suggestion ""' that in some way the different construction of the rhizome 

 is due to the horizontal position acting as a ' stimulating cause ' seems more in 

 accordance with the facts for this Selaginella. At any rate, Mr. Speakman, 

 working in my laboratory, has found that in Selaginella Lyalli all the lateral 

 branches of the rhizome become polystelic, but, while this is maintained in the 

 erect shoots, those which grow into horizontal branches become solenostelic by a 

 fusion of the ring of separate steles first developed. Bruchman also found that 

 polystelic shoots laid horizontally on the soil grow on into solenostelic rhizomes. 

 This is so fundamentally different from the relation of the solenostelic and 

 dictyostelic condition in ferns as to suggest that the homoplastic resemblance 

 is here probably not a homology of organisation but due to different factors. 



There is ground for suspecting the anatomical method when it stands by itself 

 and also for very critically considering explanations of structural changeon the 

 ground of utility. When in the attainment of any more complex whole (as, for 

 instance, in the origin of the shoot, the ovule or the flower), a new system of 

 relations is established and the external developmental morphology of the 

 primordia and their mutual relations are changed, this will be reflected in the 

 vascular system. The resulting change in the latter may, however, be profound 

 and not appear as a gradual modification of the preceding vascular system. 



In the numerous theories of the construction of the shoot the evidence relied 

 on has been partly comparison of mature form, partly, though to a less extent, 

 development, partly the vascular anatomy, and largely phyletic series, most of 

 which are very questionable. I must touch on this subject, but do not propose 

 to involve myself in the details of particular theories. They can be broadly 

 divided into those which regard the stem as in one way or another built up of 

 potentially and phyletically independent segments or phytons and those which 

 regard the shoot as a phyletically pre-existing axis or stem from which the 

 leaves have arisen by enation. 



Considering the antagonism between these two lines of interpretation orf the 

 shoot that has held throughout the history of botany, I feel diffident in 

 suggesting that there is much truth in both. In the light of general comparison 

 the leaves cannot be looked on as mere enations from an unsegmented axis ; 

 nor, on the other hand, can the latter be regarded as composed of united 

 leaf-bases. There seems an element of truth in the idea that the shoot 

 can be analysed into segments composed partly of stem and partly of leaf ; 

 and another element of truth in the idea of a central column being clothed 

 by some sort of pericaulome, though the form in which Celakovsky and Potonie 

 have stated their theories and the evidence advanced makes their accept- 

 ance impossible. While the straightforwardness of the distinction between 

 stem and leaf as put forward by Braun or in the strobilar theory is attractive 

 and lias apparent support in the ayiiral development of the higher plants, it 



"^ (i.oiialii of Bntnny, vol. xxv. I). 567. 



