340 ' LAW OF LOSS ' [CH. 



illustration of the way in which the development of the * pseudo- 

 lamina * may have occurred in the course of phyletic history. 



The phyllode theory has met with lively opposition at the 

 hands of Goebel 1 . He discusses the question chiefly in con- 

 nexion with Sagittaria^ and takes the view that the band-like 

 submerged leaves of this plant are not reduced leaves in which 

 the lamina has disappeared, but rudimentary leaves in which no 

 differentiation of blade from petiole has occurred. He supports 

 this view by recalling that, in the ontogeny of the individual 

 arrow-head leaf, stages are passed through corresponding, firstly, 

 to the band-shaped submerged leaf, and secondly to the oval 

 floating leaf. It is true that these developmental facts are not 

 easy to reconcile precisely with the phyllode theory as enun- 

 ciated by de Candolle, but they fall readily into place when 

 considered in the light of Henslow's extension of de Candolle's 

 view. If the blade of Sagittaria be merely the expansion and 

 development of the apical region of the petiole, the band-shaped 

 leaf is indeed, as Goebel says, comparable with a complete air- 

 leaf and not merely with its petiole. Where Henslow would 

 part company with Goebel would be in regarding both the 

 simple band-leaf and the highly differentiated air-leaf as homo- 

 logous with the leaf -base and -petiole alone of a typical Dicoty- 

 ledon. 



The present writer had felt for many years that it ought to be 

 possible to apply anatomical evidence to the phyllode theory, 

 and at length a path leading in this direction was disclosed. 

 Solereder 2 , in the course of a general anatomical study of the 

 Hydrocharitaceae, reported the discovery of vascular bundles 

 of inverted orientation in the leaves of various members of the 

 family (Fig. 28, p. 46). He compared the structure thus re- 

 vealed to that of petioles, Acacia phyllodes and Iris leaves, but 

 he did not, apparently, attach any theoretical importance to it. 

 It seemed, however, to the present writer that these inverted 

 bundles were an indication of the phyllodic nature of the leaves 

 in question. In the light of this idea, a general examination of 

 1 Goebel, K. (1891-1893). 2 Solereder, H. (1913). 



