102 RELATIONSHIPS OF SYMMETRY 



in the amphigastria, the insertion of which is not changed, asymmetry 

 is not observed, nor is it seen in lateral leaves which are not displaced 

 from their transverse insertion. In the special part of this book the 

 position of the leaves on the sexual shoots in which anisophylly entirely 

 disappears will be discussed. A comparative consideration shows that 

 the dorsiventral construction of the vegetative shoot must be regarded 

 as standing in direct relationship to light. The shoots of Mastigobryum 

 trilobatum which function as ' rhizophores ' have three rows of leaves 

 of equal size with a transverse insertion, and they grow away from the 

 source of light and are not plagiotropic. If one of these be cut off 

 and be allowed to continue its growth in light it will pass over into the 

 ordinary strongly anisophyllous shoot. Other examples show that 

 anisophylly in the dorsiventral liverworts is a secondary phenomenon of 

 adaptation, and this is of importance in the phylogenetic consideration 

 of the group. I may only mention one other instance. Calypogeia 

 Trichomanes has dorsiventral shoots clinging to the surface of the 

 ground, and bearing very small, often reduced, amphigastria. These 

 shoots become orthotropous when they produce gemmae, and then the 

 amphigastria become larger and are hardly less in size than the lateral 

 leaves although previously they were far surpassed by them. 



The few isophyllous Jungermannieae are orthotropous, for example, 

 Calobryum and Haplomitrium, but it appears, as I have before now 

 remarked, that Calobryum may become sometimes plagiotropous and 

 at the same time also anisophyllous, as is the case in a species of 

 Selaginella about which I shall presently speak. 



C. PTERIDOPHYTA 



Examples of anisophylly are unknown in the Filicineae and Equisetineae. 

 Its absence from the Equisetineae need not surprise us as the leaves in this 

 class are not assimilating organs and anisophylly stands, as we have seen, 

 in relation to the assimilative capacity dependent upon light. Heteropkylly, 

 which occurs in many forms and will be spoken of later, does not call 

 for consideration here, as it relates to a qualitative, not a quantitative 

 difference in the leaf-formation *. 



Some Lycopodineae, however, have to be mentioned here. In some 

 species of the genus Lycopodium, for example Lycopodium clavatum 

 and Lycopodium inundatum, plagiotropous creeping shoot-axes are found, 

 which show indeed, so far as the size of the leaf is concerned, no dorsiven- 

 trality but do so in the arrangement of the parts of their vascular bundle- 



1 This is so in the typical cases at least ; but anisophylly may also be expressed in qualitative 

 differences in leaves, as will be shown in the case of some Urticaceae. 



