338 ' LAW OF LOSS ' [CH. 



that in many cases reduction may have gone yet further, so that 

 the leaf-base alone is represented. 



The phyllode theory is supported by the existence of a 

 number of examples among Dicotyledons, in which organs not 

 dissimilar to typical Monocotyledonous leaves can be shown 

 to be equivalent either to leaf-bases, or to leaf-bases and petioles. 

 Such cases are numerous and familiar those in which the 

 reduced leaves correspond to leaf-bases alone, being decidedly 

 the commoner. In Cabomba caroliniana}, to take an instance 

 from among aquatics, two or three pairs of lanceolate simplified 

 leaves with no laminae are followed by transitional forms in which 

 a lamina occurs but is much reduced. These are succeeded by 

 the normal submerged leaves with finely divided laminae. 



It is a commonplace of every text-book that one of the most 

 distinctive features of Monocotyledons is the parallel venation 

 of the leaves. But no theory hitherto propounded regarding 

 the origin of Monocotyledons has offered any satisfactory 

 explanation of this well-marked character of the Class. To the 

 present writer it appears that one of the chief merits of de Can- 

 dolle's theory is that it explains the parallel venation of Mono- 

 cotyledonous leaves in a perfectly unstrained way. For some 

 form of parallel veining is one of the most obvious characters 

 of Dicotyledonous leaf-bases, petioles and phyllodes. Hence, 

 on de Candolle's theory, the venation of the Monocotyledonous 

 leaf ceases to present any problem; it shows precisely those 

 characters which might have been anticipated from the morpho- 

 logical nature of the organ. 



So far we have only considered those Monocotyledonous 

 leaves in which no lamina is differentiated, but we must now 

 return to the question with which we started what are the 

 homologies of the lamina in the Alismaceae and other families 

 with a corresponding foliar morphology? If the Monocotyle- 

 dons are monophyletic, two explanations are open to us; it is 

 either a revival of the lamina as it occurs among the Dicoty- 

 ledons, or an organ which has arisen de novo as a modification 



1 Raciborski, M. (18942). 



