THE PRESIDENTIAL ADDRESS. 7 



of current physiology of the higher plants, it is one of adaptive progress. 

 Accordingly, measurements must be made of the wood of fossils as well as 

 of living plants, and of young sporelings as well as of the adult. 



We have seen that plants are essentially accumulators of material. A 

 natural consequence of this is that primitive types, endowed with apical 

 growth but with no secondary cambium, will enlarge from the base 

 upwards. Any sporeling fern shows this. The leaves themselves increase 

 in number ; each successive leaf is as a rule larger than the one that came 

 before, and the stem that bears them also expands upwards. In fact, it 

 takes the form of an inverted cone. To grasp the size-problem for primi- 

 tive plants the mind must be rid of the idea of the forest tree, with its 

 stem tapering upwards, for that is a state of highly advanced organisation. 

 The primitive form of stem is that of an inverted cone, enlarging upwards, 

 with a solid core of wood within. A cone standing upon its tip is obviously 

 unpractical. Not only is it mechanically unstable, but if the original 

 structure be maintained so that the larger region above is structurally 

 a mere magnified image of the smaller below, a constantly diminishing 

 proportion of presentation-area to bulk must needs follow, in respect of 

 all the limiting surfaces. Such stems would all tend to become physio- 

 logically insufficient. Our immediate problem is with the woody column. 

 How can that due proportion of presentation-surface of the dead wood 

 to the living cells, which physiologists hold to be essential, be maintained 

 in the expanding stem, so as to meet the increasing requirements of transit 

 and distribution of the sap ? 



This is not the place for a recital of the details of elaboration of the 

 wood which have been observed and measured. It must suffice to state 

 in general terms how primitive woody plants have met the difficulty in 

 the absence of cambial thickening. The starting-point is a minute 

 cylindrical strand composed of dead tracheids only. Some primitive 

 types show nothing more than a conical enlargement of this upwards, 

 with the cells more numerous than before. The approach of a locomotive 

 at speed along a straight track may visually suggest such increase in size 

 without change of form ; successive photographs of it might be compared 

 with successive sections of those simple stems enlarging upwards without 

 change of plan. The largest examples of this are found in some of the 

 early club nmses and ferns, in which there is an enlarging solid woody 

 core. • But for want of resource in this and other features they have paid 

 the penalty of death. Most plants having this crude structure are known 



