CONTACT-PRESSURES. 237 



ever, is the determination of the ultimate value of contact-pressures 

 when these do obtain. 



From the general hypothesis of a uniform rate of growth in 

 centric systems, it follows that all contact-pressures may be 

 resolved into components acting along the orthogonally inter- 

 secting construction lines of the system ; and so long as growth is 

 uniform, no displacement can ensue, the only result being a change 

 of form ; the lateral members being, in fact, squeezed into the 

 shape of quasi-squares. That contact-pressures may exist between 

 growing primordia is undoubted, and that contacts are made in a 

 " concentration-system " : these are facts of observation. But it 

 does not follow that they are in any way pre-eminently important 

 in producing any displacements whatever in the developing 

 system. 



All theories of the effect of contact-pressures imply that the 

 primordia just formed by the growing apex exert an influence, 

 whether of the nature of a direct mechanical pressure or an 

 " induction " (Weisse), on the centre which gave them birth. That 

 such secondary centres of admittedly limited growth should thus 

 impress their individuality on the parent centre of unlimited 

 growth activities and control its subsequent operations appears at 

 first sight somewhat preposterous ; but this view has appealed 

 to many botanists, and however much such a standpoint may be 

 regarded with suspicion, since it represents an ideal post hoc ergo 

 propter hoc type of argument, the essential point is to see how 

 such a conception may have been treated from a physical or 

 mechanical standpoint, and further, what may be deduced from it. 

 It is clear to begin with that the amount of a contact-pressure 

 cannot be estimated by the eye alone, and yet observations of 

 effects which may or may not be due to such pressures constitute 

 the only means of tracing such a theory. How shallow such 

 interpretations may be is well seen, for example, in a criticism 

 of Winkler by Weisse * in which a three-angled apex is said to be 

 clearly due to the pressure exerted on it by three leaves which 

 have been just produced from it, and are naturally moving away 

 from it with the continued expansion of the growing-point. Nor 

 * Prings. Jahrh., 1903, p. 413. 



