134 THEORY OF THE STROBILUS 



independently in quite a number of series," 1 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 enquire how the 

 case stands as to the origin of foliar differentiation in Vascular 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 experiment 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 comparison, 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 present 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 which may possibly have been 

 effective in the origin of a foliar differentiation of the shoot, in any pro- 

 gressive line of evolution of vascular sporophytes: (i) 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 con- 

 struction 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 pre-existed, 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, vix. that the prototype of the leaf 

 existed from the first, and was indeed the predominant part in the initial 

 composition of the shoot, has been held by certain writers as the basis of 

 origin of the leafy shoot in vascular plants. 2 On this view not only is the 



1 Organography, p. 261. 



2 Goethe, "Die Metamorphose dor I'flanzen." Gaurlichaud, A/t'iii. de f Acad. d. Sc'i., 

 1841. Kienitz Gerloff, Bot. Zeit., 1875, p. 55. Celakovsky, " Unters. ueber die Homo- 

 logien," Pringsh. Ja/ird., xiv., p. 321, 1884; Bot. Zeit., 1901, Heft, v., VI. 



