October 28, 1915] 



NATURE 



247 



duction of a '"branch-trace" connecting with the 

 adaxial face of the subtending leaf-trace. In Helmin- 

 rhostachys a similar connection is established with the 

 --tele of the main stem, and the influence may extend 

 :u 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 forth 

 m bv experimental interference, since they occurred in 

 I plants the normal apical growth of which had been 

 • arrested. Plants of Osmunda are normally un- 

 branched, and no indication of dormant lateral apices 

 has ever been detected. I tried on young plants of 

 Osmunda regalis the experiment of injuring or de- 

 stroying the ape.x 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 

 Hotrychium and in some species of Zygopteris. The 

 disturbance of the normal growth had apparently 

 brought out (in more or less irregular form) the 

 >vstem of relations governing the position of develop- 

 ment of lateral branches. The result showed the 

 correspondence with what is the normal condition in 

 ^ome Zygopterideae. It has been said from the 

 phyletic side, and on the whole rightly, that experi- 

 ment cannot reconstruct history. In the light of Dr. 

 Kidston and Professor Gwynne-Vaughan's conclusions 

 as to the derivation of the Osmundaceae from a Zygo- 

 pterid ancestry, this induced branching of Osmunda 

 might almost be cited as a partial exception to the 

 statement. 



These examples will suffice to indicate the justifica- 

 tion 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. The problem gains in 

 interest, new questions (which are different from, 

 though not antagonistic to, phyletic problems) can be 

 put as to stelar structure, leaf-trace structure, the 

 venation of leaves, etc. We see this if we glance at 

 ^^^ the progression in stelar structure that accompanies 

 ^^^the development of the young fern. The phyletic 

 ^^■explanation has been recapitulation. We have found 

 ^^Kreason to criticise the adequacy of this as applied to 

 ^^■external form, and the same line of criticism applies 

 ^^Ko the stelar progression. In this also the early 

 ^^Bstages may be hurried over or absent, and, still more 

 ^^Beignificant, the early type of stelar structure may 

 ^^Brecur, when the shoot has fallen upon evil days and 

 ^^f approximated in size of stem and leaf-form to the 

 i-flp 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 pro- 

 gressions from protostely to a medullated monostele, 

 and from protostely to solenostely and dictyostely may 

 be treated as problems in the expansion and condensa- 

 tion of a stelar structure, which is itself the resultant 

 of a system of influences. Such parallel progressions 

 are before us within the ferns and also in other groups 

 of vascular cryptogams. 



(Alternation of generations, the morphology of the 

 seed, and the morphology of the embryo of seed-plants 

 were then considered from the viewpoint of causal 

 morphology.) 



Conclusion. 

 I have touched on a number of large questions, any 

 one of which demanded separate treatment. My con- 

 cern has not, however, been with them individually, 

 but as cognate problems justifying the deliberate adop- 



tion of a causal explanation as the aim of morpho- 

 logical work. I have confined myself to problems 

 bearing on the development and self-construction of 

 the individual, and tried to treat them so as to illus- 

 trate the causal attitude and possible lines of attack. 

 Preliminary speculations on the questions considered 

 can at best contain a germ of truth, and must be 

 subsequently adjusted in the light of further facts. 1 

 have discussed these questions rather than the smaller 

 modifications in individual development shown in 

 metamorphosis, partly because the latter have of late 

 years been treated from a causal point of view, and 

 partly because I wished to consider questions that 

 immediately affect us as working morphologists. 



Did time allow, we should naturally be led to 

 recognise the same change of attitude in biological 

 science toward the problems of the origin of new 

 forms. Questions of bud-variation and mutation are 

 clearly akin to some of those considered, and the 

 whole subject of genetics is a special attempt at a 

 causal explanation of form and structure and the 

 resulting functions. Close co-operation between the 

 morphological analysis of the plant and the genetic 

 analysis attained by the study of hybridisation is most 

 desirable. It is especially desirable that both should 

 deal with structure as well as with form, and in the 

 light of individual development. 



The causal factors which have determined and 

 guided evolution can be naturally regarded as an 

 extension of the same line of inquiry. The Darwinian 

 theory, and especially the exposition of the principle 

 of natural selection, was the greatest contribution 

 ever made to the causal explanation of the organic 

 world. Strangely enough, it led to a period of 

 morphological work in which the causal aim was 

 almost lost sight of. Why evolution has taken place 

 in certain directions and not in others is a problem 

 to the solution of which causal morphology will con- 

 tribute. The probability of orthogenesis, both in the 

 animal and vegetable kingdoms, is again coming into 

 prominence, however it is to be explained.^ When we 

 consider the renewed activity in this field it is well to 

 remember that, just as is the case with causal morpho- 

 logical work, we are picking up a broken thread in 

 the botanical web. Lastly, as if summing up all our 

 difliculties in one, we have the problem of adaptation. 

 In attacking it we must realise that use and purpose 

 have often been assumed rather than proved. We 

 may look to scientific ecological work to help us to 

 estimate the usefulness or the selection value of 

 various characters of the plant. On the other hand, 

 causal morphology may throw light on whether _ the 

 "adaptation" has not, in some cases at least, arisen 

 before there was a "use" for it. The hopeful sign 

 in the recent studv of these greater morphological 

 problems is that ' the difficulties are being more 

 intensely realised, and that rapid solutions are justly 

 suspect. The more the causal attitude is adopted in 

 ordinarv morphological work, the more hope there is 

 of these larger questions being inductively studied 

 rather than argued about. 



The causal aim is essentially different from the his- 

 torical one, but there is no opposition between causal 

 and phyletic morphology. They are rather mutually 

 helpful, for there has been an evolution, not of mature 

 plants, but of specific substances exhibiting develop- 

 ment. A deeper insight into the nature of ontogeny 

 is thus bound to be of assistance to phvletic morpho- 

 logy, while the tested results of phyletic work afford 

 most valuable guidance in general causal morphology, 

 though this cannot accept any limitation to single 

 lines of descent in its comparisons. 



I have tried to bring before you the possibilities of 

 causal morphology partly because the same attention 



NO. 24OG, VOL. 96] 



