8 THE PRESIDENTIAL ADDRESS. 



only as fossils, and no really large vascular plant lives to-day which 

 shows it. Under present conditions it is only where the size is small that 

 a simple mass of dead tracheids seems to be ejSective for water-transit. 

 Thus we see that simple enlargement without change of form does 

 not suffice. 



In more resourceful plants a remedy is found in elaboration of the form 

 and constitution of the primary wood. The changes which actually 

 appear in it, as the size of the individual or of the race increases, are very 

 various, but they all tend towards making the wood a living whole. The 

 most efficient state would be that in which each dead woody cell or element 

 faces upon one or more living cells, and this structure is approached in 

 modern types of wood. In tracing the steps which have led towards it, 

 whether in the fossil story or in the individual life of plants, we follow up 

 an evolutionary history of high functional import. Actual measurements 

 and calculations have shown in living plants the advantage that follows. 

 It has been found that changes in the elaboration of form and structure 

 of the primary woody column have saved, in specific instances, about 

 50 per cent, of the contingent loss in that proportion of presentation- 

 surface to living tissue which would have followed if a simple cylindrical 

 core had been retained. The structural changes do not, it is true, maintain 

 the full original ratio of surface to bulk, but it may well be that saving 

 even half of the contingent loss would bridge the acute risk and lead 

 to survival. 



The moulding and subdivision of the primary conducting tracts as a 

 whole, or of the woody masses which they contain, present the most varied 

 features. Their contours often appear arbitrary and even irrational, 

 so long as no underlying principle is apprehended. They have presented 

 a standing problem to anatomists. But when it is realised that as the 

 size increases there is a physiological advantage in any elaboration of 

 form whatsoever, a rational explanation is at hand. The variety of the 

 forms assumed suggests the common principle underlying them all, which 

 is that thereby a due proportion of presentation-surface tends to be 

 maintained. 



One of the simplest and most frequent examples of such elaboration 

 of form is that of the fluted column, which in transverse section gives the 

 familiar stellate figure characteristic of roots. It is also seen in many 

 stems, and is described as ' radial.' Where the part is small the woody 

 strand is roughly cylindrical, but where larger it often becomes fluted 



