PLANT MORPHOLOGY 69 







upon this phytonic question by Delpino. As a consequence of his 

 studies on phyllotaxis he concluded that the axis is simply composed 

 of the fusion of the leaf-bases : that the leaves are not appendicular 

 organs, but central organs: that an axis or stem-system does not 

 exist, and accordingly that the higher plants are not cormophytes 

 at all, but phyllophytes. There will, I think, be few who will adopt 

 this fantastic view of the shoot. 



The second view, that the axis and leaf are the result of differentia- 

 tion of an indifferent branch-system, of which the limbs were origi- 

 nally all alike, has lately been brought into prominence by Potonie". 1 

 Taking his initiative from the branching of the leaves in early fossil 

 ferns, he recognizes the frequent occurrence of overtopping (Ueber- 

 gipfelung), that is, the gradual process of assertion of certain limbs 

 of a branch-system over others: in the branching of fucoids he 

 finds an analogy for his observations on fern-leaves, and draws the 

 following conclusion: that "the leaves of the higher plants have 

 been derived in the course of generations from parts of an Algal 

 thallus like that of Fucus, or at least from Alga-like plants, by means 

 of the overtopping of dichotomous branches, and the development 

 as leaves of the branches, which thus became lateral." Dr. Hallier, 

 who adopts Potoni6's position, prefers to draw the comparison with 

 liverworts, which show a similar sympodial development of a dicho- 

 tomous branch-system. 2 



It seems not improbable that the condition of many branched 

 fern-leaves may have been derived through a process of "over- 

 topping" in an indifferent branch-system of the leaf itself. But it 

 lies with Potonie to show, on a basis of comparison of forms more 

 nearly related to them than the fucoids, that the relation of axis 

 to leaf in the ferns was so derived : and further, that such an origin 

 is in any way applicable to other foliar developments in vascular 

 plants, especially pteridophytes, such as the lycopods, equiseta, and 

 sphenophylls. I am not aware that this has yet been done. But. 

 granting that this can be done, the question still remains whether 

 similarity of method of branching is any criterion of comparison as 

 to descent? And especially whether such comparison is valid be- 

 tween widely distinct groups, or between the different generations 

 of an antithetic alternation? It is true that Potonie prefers to regard 

 such generations as homologous, as is indeed essential for his view: 

 but that does not prevent others from differing from him, or even 

 considering the fact that the parts compared belong to different 

 generations as fatal to his theory. For my own part I am not prepared 

 to give up the broad conclusions as to antithetic alternation on so 



1 Lehrbuch der Pflanzenpalceontologie, pp. 156-159. Also Ein Blick in die Ge- 

 schichte der Bot. Morph. und der Pericaulomtheorie, 1903, p. 33, etc. 



2 Beitrage zur Morphogenie der Sporophylle und des Trophophylls. Morph. d. 

 Sporophylle u. d. Trophophylls, Hamburg, 1902. 



