POLYPHYLETIC ORIGIN OF LEAVES 



133 



morphological method of the latter half of the nineteenth century, all the 

 parts which share these characters would rank as " leaves," and be regarded 

 as "homologous." But the progress of the science should be leading 

 towards the refinement of the use of the term " homology " : an approach 

 must be made, however distant it may yet be, to a classification of parts 

 on a basis of Descent. Though this may readily be accepted in theory, 

 it is still far from being adopted in the practice of Plant-Morphology. 

 None the less, comparison is inevitably leading to the disintegration, on a 

 basis of Descent, of the old-accepted categories of parts : and in the case 

 of the appendages which are collectively styled " leaves," the question arises 

 whether they are all truly comparable in Vascular Plants. This is clearly 

 in close relation to the question of their origin, and we shall enquire 



FIG. 71. 



Longitudinal section through the apical cone of the stem of Lycopodiutn Sclago. X 160. 

 (After Strasburger. ) 



whether there is not reason to think that the initiation of the foliar 

 appendages may have been polyphyletic. 



To those who hold the view that the two alternating generations of 

 the Archegoniatae have had a distinct phylogenetic history, it will be clear 

 that their parts can not be truly comparable by descent. The leaf of the 

 Vascular Plant, accordingly, will not be the correlative of the leaf of a Moss. 

 Even those who regard the sporophyte as an unsexed gametophyte will 

 still have to show, on a basis of comparison and development, that the 

 leaves of the two generations are of common descent. I am not aware 

 that this has yet been done by them. 



But the phylogenetic distinctness of origin of the leaves of the sporo- 

 phyte and gametophyte is not the only example of parallel foliar develop- 

 ment. Goebel has shown with much cogency that the foliar appendages of 

 the Bryophytes are not all comparable as regards their origin ; he remarks, 

 " It is characteristic that the leaf-formation in the Liverworts has arisen 



