THE PRESIDENTIAL ADDRESS. 9 



with varying number and depth of the flanges. In many instances the 

 ratio of their number to the diameter of the whole tract is approximately 

 constant. The structure is in fact adjusted to the size. This is so in roots 

 generally, in leafy stems and in leafless rhizomes — and a similar size- 

 relation is even found in the fluted chloroplasts of certain Algae. In all of 

 these an obvious risk following an increase in size tends to be eliminated, 

 viz., an undue loss of proportion of surface to bulk. 



The somewhat technical facts thus briefly described may be taken as 

 examples of a relation of form to size which is very general. They suggest 

 the existence of a * size-factor,' which is effective in determining form. The 

 susceptibility to its influence resides in the part that shows the results. 

 The internal contours are defined ah initio, instead of coming into existence 

 during the course of development, as is the case with the convolutions 

 of the mammalian brain. In the stem and roots of vascular plants the 

 fully- matured conducting tracts may be traced upwards, with their outlines 

 already defined, through successive stages of youth towards the growing 

 point, which has been their source. Their form may be seen already 

 outlined in its young tissue closely short of the extreme tip. This fact 

 suggests that the susceptibility to the size-factor resides in the growing 

 point itself, for immediately below it those tracts possess that form which 

 will aid their function when they are fully developed. 



Of all the factors that contribute to the determination of form in 

 growing organisms there is none so constant and inevitable in its incidence 

 as this size-relation. Its operation becomes manifest with the very first 

 signs of differentiation of the embryonic tissues. The effects of other 

 factors that influence form, such as gravity, light, temperature, contact 

 and the rest appear later in point of time. Their influence is liable to 

 diminish as the organism reacts to them by curvature or otherwise, and 

 to vanish when the reaction is completed. Under experiment they may 

 be controlled or even inhibited. But the operation of the size-factor is 

 insistent ; it cannot be avoided either under conditions of nature or by 

 experiment, though the size itself may be varied under conditions of 

 nutrition and the permeability of the presentation-surfaces may not be 

 constant, with results as yet unknown. When we reflect that all acquisi- 

 tion of nourishment and transit of material in plants of primary construc- 

 tion is carried out through limiting surfaces, the essential importance of 

 the size-factor is evident, for upon its influence the proportion of each 

 presentation-surface itself depends. 



