10 THE PRESIDENTIAL ADDRESS. 



The evidence that size itself is, among other factors, a determinant of 

 form rests upon the constancy with which, in an enlarging organism, 

 changes of primary form tend to maintain a due area of presentation- 

 surface such as active transit demands. That evidence has been derived 

 chiefly from the conducting tracts of primary individuals as they enlarge 

 conically upwards, and from parts belonging to distinct categories, also 

 from comparison of different individuals not necessarily of close alliance. 

 Very cogent evidence lies in the variety of the changes of form by which 

 the same end is attained. Finally, the converse facts bring conviction 

 when, as often happens, a distal diminution of size in stem or leaf is 

 accompanied by simplification along lines roughly the converse of those 

 that follow increase. All this shows that a real relation exists between 

 size and primary form. The term ' size-factor ' has been used to connote 

 that influence which affects form in relation to size, but without defining 

 it except by its results. Nevertheless, we have seen that its action may 

 be located in near proximity to the growing point, or in the embryo itself. 

 It has not, however, been found possible to assign to that effect an 

 immediate cause. The attitude thus adopted towards an undoubted 

 factor seems justified by the broad logic of science, and by the practice of 

 its highest votaries. When Newton put together his great physical 

 synthesis he pointed out at the close of the Principia that the cause of 

 gravitational force was unknown. ' Hitherto I have not been able to 

 discover,' he said, ' the cause of these properties of gravity from 

 phaenomena, and I frame no hypotheses.' Likewise, in its own more 

 restricted field of botanical phenomena, the size-factor may be recognised 

 as effective in development, though the immediate cause of its effectiveness 

 is still unknown. 



The position thus adopted assumes the shoot to be a unit, not a 

 congeries of ' phytons.' The elaboration of its form, whether external or 

 internal, would be a function of the increase in size of that unit, and the 

 result would tend to maintain the adequacy of the presentation-surfaces. 

 This conception of the shoot and of its parts would accord with the views 

 of General Smuts, as stated in his remarkable work on ' Holism,' published 

 in 1926. Many present here to-day will have heard his Address in Cape 

 Town last year, when opening the discussion on ' The Nature of Life.' 

 All will value this masterly statement in brief of his theory. I suggest 

 that the operation of the size-factor, whether in relation to external leaf- 

 development or in the elaboration of internal conducting tracts, illustrates 



