344 ' LAW OF LOSS ' [ch. 



It may be worth noting that a pecuHar submerged member 

 of this family, Hydrothrix Gardneri^ Hook, f., described by 

 Goebel^, has leaves with a sheathing base and hair-like upper 

 region, whose external morphology distinctly suggests a 

 phyllodic origin. In this case anatomical evidence cannot be 

 sought, since the extremely slender leaves are said to be tra- 

 versed by a single bundle only. 



The presence of inverted bundles in all species of Pontederia- 

 ceae of which material has been available to the present writer, 

 is a remarkable anomaly which calls for some explanation. It 

 is difficult to see how such a structural peculiarity can be ex- 

 plained as an adaptation, since it is common to leaves otherwise 

 differing notably in type and mode of life. It is equally con- 

 spicuous in the very delicate ribbon-leaf of Heteranthera zos- 

 teraejolia and in the well-defined, thick * lamina ' of Eichhornia 

 speciosa\ it occurs both in Heteranthera reniformis, in which 

 palisade parenchyma is confined to the upper side and in 

 Pontederia cordata^ in which this tissue occurs towards both 

 surfaces. In the present writer's opinion, this anatomical ano- 

 maly is best interpreted on the view that the ' laminae ' of the 

 Pontederiaceae, instead of being homologous with the blades 

 of Dicotyledons, are merely the expanded apices of pre- 

 existing phyllodes : the inverted bundles are thus an indication 

 of the petiolar nature of the organ, and are regarded as an 

 ancestral feature rather than as an adaptation. 



The Pontederiaceae are not the only family in which we 

 meet with phyllodic anatomy of the 'lamina.' The present 

 writer has found, in the arrow-head blade of Sagittaria monte- 

 vidensis^ Cham, and Schlecht. (Fig. 171 B\ that, besides the 

 normal main bundles (n.b^ and a series of smaller bundles 

 running near the lower surface {n.b^^ there is a third series 

 of small inverted bundles near the upper surface [i.b^. In 

 Sagittaria sagittijolia^ L., inverted bundles are a less striking 

 feature, but the lateral ribs, one of which is represented in 

 Fig. 171 ^, show both normal and inverted bundles. 



1 Goebel, K. (19 13). 



