66 PLANT MORPHOLOGY 



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

 ment; Goebel has shown with much cogency that the foliar append- 

 ages 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 " 

 (Organographie, p. 261), and has shown that they must have been 

 produced in different ways. Here, then, is polyphyleticism in high 

 degree, 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 inquire into 

 the views current as to the origin of foliar differentiation in vas- 

 cular plants. In discussing such questions it is to be remembered 

 that in different stocks the foliar condition of the sporophyte as we 

 see it may have been achieved in different ways, just as investigators 

 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 indirect, since no intermediate types are known 

 between foliar and non-foliar sporophytes. Physiological experi- 

 ment has as yet nothing to say on the subject. The fossil history 

 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 com- 

 parison, while anatomical detail may have some bearing also on 

 the question: but all of these, as indirect lines of evidence, fall short 

 of demonstration, and accordingly it is impossible to come at pre- 

 sent 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 originated in essentially the same way, 

 though not necessarily along the same phyletic line. 



There are at least three alternatives possible for the origin of the 

 foliar differentiation of the shoot, in any progressive line of evolu- 

 tion 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 these 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 indifferent branch-system, of which the 

 limbs were originally all alike; in this case neither leaf nor axis 

 would predominate from the first; (3) that the axis preexisted, and 

 the foliar appendages arose as outgrowths upon it: in this case the 

 axis would be from the first the predominant part. 



The first of the above alternatives, viz., that the prototype of the 



