ECCENTRIC GROWTH. 281 



type of Selaginella, in which the general form relations of the 

 lateral members are indicated with a degree of accuracy quite 

 sufficient to illustrate the main principles. It may be noted also 

 that such a type, in which the leaf-members are not specialised to 

 any extent, but remain in the primary " leaf-base " type of member, 

 illustrate such simple mathematical construction relations much 

 more obviously than is to be expected in more specialised petiolate 

 forms in which the primary leaf-primordium may have undergone 

 secondary segmentation. From the construction of the diagram it 

 also follows that the greater the degree of eccentricity in the 

 construction the more marked will be the anisophylly ; so that in 

 Selaginella, for example, the difference in the size of the two pairs 

 of leaves becomes a measure of the amount of secondary divergence 

 from the primitive radially symmetrical type. 



That a shoot-system should become obviously eccentric in its 

 growth and exhibit phenomena variously included under the terms 

 " dorsiventrality," "zygomorphy," and "anisophylly," in the sense 

 defined by both Wiesner and Goebel, is, however, after all not the 

 most remarkable feature of such shoot construction ; or again that 

 such eccentricity should be possibly induced by external environ- 

 ment, whether light action, as in the case of certain foliage-shoots, 

 or as an adaptation to insect-visits, as in the case of many flower- 

 shoots. The eccentric tendency may be inherent in any shoot, and 

 especially so in lateral ones, and if at all advantageous may become 

 exaggerated by natural selection with the production of its more 

 or less marked after effects; but the most remarkable feature is 

 not the existence of the eccentricity itself, so much as the manner 

 in which the eccentric construction when markedly developed 

 becomes established with a constant orientation in the phyllotaxis 

 system. In other words, the displacement of the growth-centre 

 in such cases is not accidental, but follows a definite direction 

 which must often be accurate to a remarkable degree. These 

 phenomena include both the orientation of the foliage of the 

 " dorsiventral " shoot and the "plane of zygomorphy " of the eccentric 

 flower-shoot. In the case of foliage-shoots, the production of an 

 amount of structural eccentricity sufficient to appeal to the eye in 

 the anisophylly of the leaves in the case of normal asymmetrical 



