PRESIDENTIAL ADJjRESS. 700 



iiiijiwxliak'ly liojiel'iil piubleiii. Here also, in (-DnsideiiDg tlie tern, we are 

 assisted by homologies of organisation in other Vascular Cryptogams and in 

 the more complex Bryophyta, though the Algse 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 inde- 

 pendent 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 

 cur knowledge usually comes between the apical meristeni 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 differ- 

 ential. on requires separate study for each grade of complexity of the vascular 

 system even in the same plant. 



If we leok at the vascular system, not as if it had an independent existence 

 nor from the phyletic point of view, but as a differentiation taking place within 

 the body of tlie individual plant, we can inquire as to the causal factors in the 

 process.'-' A deeper insight iu.o the nature of the stele may be obtained by 

 regarding it as the resultant of a mmiber of factors, as part of the manifestation 

 of the sy.stein 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 inlluences are modified. Herein lie'-, the value 

 of abnormalities, of natural experiments, and the results of experimental inter- 

 ference. Possible influences that have at various times been suggested are 

 functional stimuli, the inductive 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 inductive 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 developing primordia. 

 We have further to take external stimuli into account, though these must act 

 by influencing 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 morpho- 

 logical 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 this 

 coincides with the cell-segmentation or not, and such a segmental construction 

 is reflected in the vascular system. Can we in the first place distinguish any 

 parts played by influences from the stem-apex and the developing leaves respec- 

 tively ? Unfortunately we 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, when 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 



'■' .Vii advantage that follows from this is that we get clear of the misleading 

 metaphor of leaf-traces 'passing out,' &c. 



