XXVI 1 1] 



PONTEDERIACEAE 



the leaves of Monocotyledons was undertaken, with the result 

 that * phyllodic ' anatomy was found to occur frequently in this 

 Class. In many cases the existence of this type of structure 

 had already been recognised, but it had not been interpreted 

 as ' phyllodic.' In other instances the existence of inverted 

 bundles had apparently been overlooked. This was the case in 

 the Pontederiaceae an aquatic family belonging to the Fari- 

 nosae; it may therefore be worth while to describe the leaf 

 structure of this group in some detail. The leaves, as a rule, 

 have a sheathing leaf-base, a petiole, which is sometimes much 



FIG. 169. A, 'lamina' of Pontederia cor data, L.; B, small 'lamina' of Eichhornia 

 speciosa, Kunth. (Reduced.) [Arber, A. (1918).] 



swollen, and a * lamina.' In external appearance and venation 

 the leaves of Pontederia (Fig. 169^ and Eichhornia (Fig. 1 69 B) 

 distinctly suggest that the * laminae ' are produced by expansion 

 of the apical region of the petiole, and that they are thus 

 * pseudo-laminae ' and not equivalent to the blades of Dicoty- 

 ledonous leaves. The anatomy confirms this idea in a striking 

 fashion. Fig. lyo-D, p. 342, shows the transverse section of a 

 petiole of Pontederia cordata, L. with inverted bundles towards 

 the upper side. When the ' lamina ' is cut transversely, its 

 structure is found to be exactly such as might have been 

 anticipated on the theory that it is produced by extreme 



