The Profound Effect on the Structure of Plants 51 



FIG. 7. The form, as seen in vertical 

 section, which a plant would display 

 (theoretically) if free to adapt itself to 

 photosynthesis alone. Further particu- 

 lars in the text. 



It is worth while to try to express the sum of these features 

 in diagrammatic form, and my suggestion thereof is contained 

 in figure 7. The purely photosynthetic plant would exhibit 

 a system of equal rigid branches 

 springing as radii from a central 

 trunk, and forking regularly 

 outward to a vast number of 

 young twigs which would turn 

 up near the tips to spread the 

 leaves horizontally in a hollow 

 hemisphere of foliage. This 

 theoretical form, of course, is 

 modified in practice by other 

 considerations, especially the 

 exigencies of mechanical sup- 

 port, as we shall later consider; 



but nevertheless it comes appreciably close to realization in the 

 most typical of the great trees, when these are free to develop 

 without interference, as was the case with the Oak of the ac- 

 companying picture (figure 8). 



We turn now to a particular study of those two most distinctive 

 plant structures, the leaf and the stem. A first view over leaves 

 in general gives only the impression of bewildering multiformity; 

 but continued observation gradually sorts out the important 

 from the trivial, and builds one of those visualized composites of 

 which I have spoken in the first chapter. As the reader should 

 review and confirm for himself by inspection of a number of 

 kinds brought together for the purpose, the principal part of an 

 ordinary leaf is the spreading thin blade, which exhibits two con- 

 stituents, first, the soft, seemingly-homogeneous, chlorophyllous 

 tissue, denser in green on the uppermost surface, and seat of the 

 food-making process, and second, the slender white veins, spring- 

 ing out from the leaf-stalk and variously branching and inter- 

 lacing while ever attenuizing towards the margin and tip of the 



