xxvm] PHYLLODE THEORY 339 



of the distal region of a pre-existing phyllode. In deciding 

 between these two alternatives, the Law of Loss comes to our 

 assistance. On this law, the blade once lost cannot be regained^ 

 and it is therefore clear that the * lamina ' of the Monocotyle- 

 don is, as Henslow 1 has suggested, an expansion of the petiole, 

 imitative of, but not identical with, the blade of a Dicotyledon : 

 the present writer proposes to distinguish such a blade as a 

 * pseudo-lamina.' This interpretation certainly accords well 



FIG. 167. Potamogeton 

 lucens, L. Apical part 

 of a shoot showing range 

 of leaf form. (Reduced.) 

 [Raunkiaer, C. (1896).] 



\ 



FIG. 1 68. Potamogeton natans, L. Series of leaf forms including 



A, the normal floating 'lamina.' (A and B, reduced; C-E, nat. 



size.) [Raunkiaer, C. (1896).] 



with the venation of many Monocotyledonous leaves. The 

 transitional leaf forms produced in Sagittaria between the band 

 and arrow-shaped types (Fig. 5, p. 14) have all the appearance 

 of merely representing different degrees of expansion of the 

 upper region of the petiole, with correspondingly varying 

 degrees of outward curvature and apical detachment of the 

 veins. A somewhat similar series can be traced in certain 

 Potamogetons (Figs. 167 and 168). These series afford an 



1 Henslow, G. (1911). 



22 2 



