io 4 RELATIONSHIPS OF SYMMETRY 



of the lateral leaves evidently fits them to serve most efficiently as flat 

 leaf-like organs of assimilation, without torsion being necessary. The upper 

 and the under leaves are also different from one another. The under ones 

 are smaller, pale, without a prominent leaf-cushion, and take no part in 

 assimilation ; the upper leaves are rich in chlorophyll and have a leaf- 

 cushion, and this cushion adds to the assimilating surface although not 

 to any very great extent. The difference in the formation of the leaves 

 is not yet visible in the vegetative point although the dissimilarity 

 between the upper and the under leaves may appear very early 

 (Fig. 56, i). 



The subterranean shoots are very different in configuration from these 

 assimilating shoots. They are radial with spiral phyllotaxy, nearly cylin- 

 dric in transverse section, bear leaves which are all constructed alike, 

 and have a system of vascular bundles which is radial, whilst that of the 

 dorsiventral shoots is bilaterally symmetric (see Fig. 56, 2). The 

 branching of the subterranean shoots, too, does not take place in one 

 plane, and thus differs from that in the aerial shoots. Between the two 

 forms of shoots there are all intermediate stages. The shoots which raise 

 themselves above the ground only gradually attain dorsiventrality and 

 anisophylly. Primarily they are radial with leaves in many rows ; then they 

 flatten upon the side turned towards the light but remain convex upon 

 the shaded side, and the leaves upon the flanks now take on the keel-like 

 form. The branching then proceeds in one plane, and this shows that 

 dorsiventrality is become impressed upon that shoot. Upon the upper 

 parts of the branches thus formed there arise the shoots of higher order 

 described above, which are strongly dorsiventral and have the characteristic 

 arrangement of leaves of this form. My researches have shown that the 

 dorsiventrality as well as the anisophylly is here conditioned by light. 

 In this history we observe then the gradual engrafting of both features, 

 and that shoots of a higher order react more strongly in response to the 

 external determining factor than do those which spring as lateral branches 

 directly from the rhizome ; at the same time it is of extreme interest to 

 see how within one cycle of affinity, that namely of the Lycopodieae, 

 the problem of making a shoot with four rows of leaves into a plagio- 

 tropous dorsiventral anisophyllous one is solved in different ways. This 

 is accomplished in Selaginelleae by quite other means. In Lycopodium 

 complanatum the four-rowed phyllotaxy arises by reduction ; whether 

 such a process takes place in the Selaginelleae we leave undecided, as we 

 have here only to do with observed relationships ; at any rate the reduction 

 in the Selaginelleae with four rows of leaves must have taken place only 

 in the course of pJiylogeny, not in the ontogeny. The differences in the 

 point which is reached by the same path show us again that in all 

 adaptations we must consider not only the influence of the environment, 



