Decembeb 3, 1915] 



SCIENCE 



789 



mon construction. The relation that exists 

 between the general construction and the 

 vascular anatomy offers a special and 

 more immediately hopeful problem. Here 

 also, in considering the fern, we are as- 

 sisted by homologies of organization 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 con- 

 cerned and the fact that they have been 

 almost as readily studied in fossil as in 

 living plants has led to this. Not less im- 

 portant 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 morphol- 

 ogy, and anatomy has adopted further con- 

 ventions of its own and tended to treat the 

 vascular system as if it had an almost in- 

 dependent existence in the plant. The 

 chief method employed has been the com- 

 parative 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 ade- 

 quately studied in the light of apical de- 

 velopment. A gap in our knowledge usu- 

 ally comes between the apical meristem 

 itself and the region with a developed vas- 

 cular 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 differ- 

 entiation 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 regarding it as the resultant 

 of a number of factors, as part of the man- 

 ifestation of the system of relations in de- 

 velopment. The first step towards this is 

 the critical consideration of normal devel- 

 oping plants, but so long as the causal in- 

 fluences in the developing substance of a 

 plant remain unchanged the resulting vas- 

 cular structure will remain constant. Our 

 hope of advance lies in the study of eases 

 where these influences are modified. Herein 

 lies the value of abnormalities, of natural 

 experiments, and the results of experimen- 

 tal interference. Possible influences that 

 have at various times been suggested are 

 functional stimuli, the inductive influence 

 of the older pre-formed parts on the de- 

 veloping region, and formative stimuli of 

 unknown nature proceeding from the de- 

 veloping region. The functional stimuli 

 do not come into play at the time of lay- 

 ing down the vascular tracts, though they 

 may have importance in their maintenance 

 later; the inductive influence of the ana- 

 tomy 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 influ- 

 encing the internal system of relations. 



I have touched on a number of large 

 questions, any one of which demanded sepa- 

 rate treatment. My concern has not, how- 

 ever, been with them individually, but as 

 cognate problems justifying the deliberate 



