THE PRESIDENTIAL ADDRESS. 5 



on this point that such a relation does not exist. ' This much is proved,' 

 says Prof, von Goebel, that, ' so far as we can see, the question relates to 

 conditions of growth and symmetry that arise in the growing point.' 

 ' All theories as to leaf-position that allotted a passive role to the growing 

 point were mistaken, however acute the reasoning that was brought to 

 bear thereon ' {Organographie. 3rd Edn., part I, pp. 299-300). This is 

 Von Goebel's summing up for external parts. On the other hand, within 

 the growing point, and often, though not always, related to the external 

 parts, there is a progressive formation of internal conducting tracts, 

 continuous from the adult region upwards to the tip. A like reference of 

 the origin and disposition of these vascular tracts to the growing point 

 itself appears to be equally justified. In fact, the tip possesses the 

 initiative for both. 



The complex shoot that results from such initiation is exposed as it 

 matures to external conditions which modify its form. Their effect is 

 very obvious in the young shoot of the higher plants. As the shoot 

 elongates its young tissues are soft and plastic. While in this state its 

 form may be influenced by gravity, the incidence of light, mechanical 

 contact and other causes which produce reactions of form called ' tropisms.' 

 All of these promote the well-being of the whole. The net result becomes 

 fixed as the part matures, and its constituent tissues harden. Thus, the 

 adult form is the consequence of the primary initiation at the growing 

 point, modified by the conditions to which the plant may have been exposed 

 during the plastic period. This is a commonplace of the text-books. 

 But amid all the careful analysis and experiment that has been devoted 

 to the influences which thus affect form, one factor, insistent and un- 

 avoidable, has been habitually left out, viz., the influence of size. 

 Reference is occasionally made in textbooks to the effect of surface- 

 tension in determining the simple form in minute organisms, such as 

 •unicellular Algae and Bacteria, and to the deviations from that simple 

 form as the size increases, and the influence of surface-tension ceases to 

 be dominant. At the other end of the scale of size mathematicians have 

 calculated the extreme stature mechanically possible for a tree-trunk 

 constructed after the ordinary plan, and of materials of known strength. 

 The result is about 300 feet, and this coincides approximately with the 

 limit of height of the canopy of a tropical forest. But in point of size 

 practically the whole of the vegetable kingdom lies between the microbe 

 and the forest tree. Unfortunately, the study of these middle terms, 



