246 



NATURE 



[October 28, 19 15 



of the shoot, behind, to one side, or on the leaf-base, 

 itself, the most interesting but least common case 

 being when the branch is in an axillary position. 

 When the mature shoot only is considered, it is pos- 

 sible to argue for the derivation of monopodial branch- 

 ing from dichotomy or the converse. Even the facts 

 obtainable from the mature plant, however, point 

 to the dichotomous branching being a modi- 

 fication of the monopodial, the opposite view- 

 appearing to land us in difficulties regard- 

 ing the morphology of the main shoot. It is 

 unlikely that a dichotomy involving the division of 

 the apical cell occurs in the fern-shoot, and com- 

 parison with the Bryophyta confirms the suspicion 

 that the cases of dichotomy are only apparent. 



In considering the construction of the shoot we are 

 at present limited to comparison of the normal 

 structure and development. The system of relations 

 in the shoot of the fern, affecting in the first place 

 the distribution of the leaves and secondly that of the 

 branches, appears, however, to be of the same nature 

 as in the independently evolved shoots of Bryophyta 

 and algae. A morphological analysis based on the 

 simpler examples may lead on to the experimental 

 investigation of the common construction. The rela- 

 tion that ekists between the general construction and 

 the vascular anatomy offers a special and more imme- 

 diately hopeful problem. Here also, in considering 

 the fern, we are assisted by homologies of organisa- 

 tion in other vascular cryptogams and in the more 

 complex Bryophyta, though the algae are of little 

 help. 



In few departments of botany has our knowledge 

 increased so greatly and become so accurate as in 

 that of vascular anatomy. The definiteness of the 

 structures concerned and the fact that they have been 

 almost as readily studied in fossil as in living plants 

 has led to this. Not less important have been the 

 clear concepts first of the bundle system and later 

 of the stele under which the wealth of fact has been 

 brought. Great progress has been made under the 

 influence of phyletic morphology, and anatomy has 

 adopted further conventions of its own and tended 

 to treat the vascular system as if it had an almost 

 independent existence in the plant. The chief method 

 employed has been the comparative study of the 

 mature regions, of necessity in the fossils and by 

 choice in the case of existing plants. I do not, of 

 course, mean to say that we are ignorant of the 

 development of the vascular system, but the variety 

 in it has not been adequately studied in the light of 

 apical development. A gap in our knowledge usually 

 comes between the apical meristem itself and the 

 region with a developed vascular system. It is in this 

 intermediate region that the real differentiation takes 

 place, and the arrangement of the first vascular tracts 

 is then modified by unequal extension of the various 

 parts. The apical differentiation requires separate 

 study for each grade of complexity of the vascular 

 system, even in the same plant. 



If we look at the vascular system, not as if it had 

 an independent existence or from the phyletic point 

 of view, but as a differentiation taking place within 

 the body of the individual plant, we can inquire as to 

 the causal factors in the process. A deeper insight 

 into the nature of the stele may be obtained by regard- 

 ing it as the resultant of a number of factors, as part 

 of the manifestation of the system of relations in 

 development. The first step towards this is the critical 

 consideration of normal developing plants, but so long 

 as the causal influences in the developing- substance of 

 a plant remain unchanged the resulting vascular 

 structure will remain constant. Our hope of advance 

 lies in the study of cases where these influences are 

 modified. Herein lies the value of abnormalities, of 

 NO. 2400, VOL. 96] 



natural experiments, and the results of experimental 

 interference. Possible influences that have at various 

 times been suggested are functional stimuli, the in- 

 ductive influence of the older pre-formed parts on the 

 developing region, and formative stimuli of unknown 

 nature proceeding from the developing region. The 

 functional stimuli do not come into play at the time 

 of laying down the vascular tracts, though they may 

 have importance in their maintenance later ; the in- 

 ductive influence of the anatomy of older regions is 

 excluded in the first differentiation of the vascular 

 system in an embryo ; we are thus led to attach 

 special importance to the detection of the action of 

 formative stimuli proceeding from the young develop- 

 ing primordia. We have further to take external 

 stimuli into account, though these must act by in- 

 fluencing the internal system of relations. 



Time will not permit of reference to the scattered 

 literature bearing on this subject, but it may make 

 the reality of such formative stimuli a little clearer 

 if I refer to some examples that have turned up in 

 the course of my morphological work. In the case 

 of the shoot, formative influences must act in the 

 small apical region where we have the meristematic 

 growing point with the primordia of the leaves. There 

 is a presumption in favour of some sort of segmental 

 construction of the meristem in relation to the leaves, 

 whether tliis coincides with the cell-segmentation or 

 not, and such a segmental construction is reflected in 

 the vascular system. Can we in the first place distin- 

 guish any parts played by influences from the stem- 

 apex and the developing leaves respectively? Unfor- 

 tunately w^e know little or nothing of the anatomical 

 relations in the rare cases of adventitious leaf- 

 formation. We get a little insight into the respective 

 influences of leaf and axis, however, w-hen we compare 

 shoots with well-developed leaves and those without 

 leaves or with greatly reduced leaves ; this may be 

 done between distinct plants or between different 

 regions of the 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 dif- 

 ferent 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 Psilotaceae, 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 in- 

 fluences 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 deterniine thfe 

 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 pro- 



