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SCIENCE. 



[N. S. Vol. XX. No. 512. 



plant morphology. None the less, com- 

 parison is inevitably leading to the dis- 

 integration, on a basis of descent, of the 

 old-aecepted categories of parts; of these 

 the most prominent, and at the same time 

 the most debatable, is the category of 

 leaves, and they will lend themselves best 

 to the illustration of the matter in hand. 



To those who, like myself, hold the view 

 that the two alternating generations of the 

 Archegoniatje have had a distinct phylo- 

 genetic history, it will be clear that their 

 parts can not be truly comparable by de- 

 scent. The leaf of the vascular plant, ac- 

 cordingly, 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 the 

 leaves in the sporophyte and gametophyte 

 is not the only example of parallel foliar 

 development. 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 independently in 

 quite a number of series' ('Organog- 

 raphie,' p. 261), and has shown that they 

 must have been produced in different ways. 

 Here then is polyphyleticism in high de- 

 gree, seen in the origin of those parts of 

 the gametophyte which on grounds of 

 descent we have already separated from 

 the foliar appendages of the sporophyte. 



Such results as these for the gametophyte 

 lead us to enquire into the views current 

 as to the origin of foliar differentiation in 

 vascular plants. In discussing such ques- 

 tions, it is to be remembered that in dif- 

 ferent stocks the foliar condition of the 

 sporophyte as we see it may have been 



achieved in different ways, just as investi- 

 gators have found reason to believe that it 

 was in the gametophyte. "We have no right 

 to assume that the leaf was formed once for 

 all in the descent of the sporophyte. But 

 at the moment we are unprovided with any 

 definite proof how it occurred. All the 

 evidence on the point is necessarily indi- 

 rect, since no intermediate types are known 

 between foliar and non-foliar sporophytes. 

 Physiological experiment has as yet noth- 

 ing to say on the subject. The fossil his- 

 tory of the origin of the foliar state in the 

 neutral generation is lost, for the foliar 

 character antedated the earliest known 

 fossil-sporophytes. There remain the facts 

 of development of the individual, and 

 comparison, while anatomical detail may 

 have some bearing also on the question; 

 but all of these, as indirect lines of evi- 

 dence, fall short of demonstration, and 

 accordingly it is impossible to come at pres- 

 ent to any decision on the point. For the 

 purposes of this discussion, however, we 

 shall proceed on the supposition that all 

 leaves of the sporophyte generation origi- 

 nated in essentially the same way, though 

 not necessarily along the same phyletic 

 line. 



There are at least three alternatives pos- 

 sible for the origin of the foliar differentia- 

 tion of the shoot, in any progressive line of 

 evolution of vascular sporophytes: (1) 

 That the prototype of the leaf was of prior 

 existence, the. axis being a part which 

 gradually asserted itself as a basis for the 

 insertion of those appendages; the leaf in 

 such a case would be from the first the 

 predominant part in the construction of 

 the shoot. (2) That the axis and leaf are 

 the result of differentiation of an indiffer- 

 ent branch-system, of which the limbs were 

 originally all alike ; in this case neither leaf 

 nor axis would predominate from th^ first. 

 (3) That the axis preexisted, and the foliar 

 appendages arose as outgrowths upon it ; in 



