5S8 



SCIENCE. 



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



course of generations from parts of an 

 algal thallus like that of Fucus, or at least 

 from alga-like plants, by means of the over- 

 topping of dichotomous branches, 'and the 

 development as leaves of the branches, 

 which thus become lateral.' Dr. Halliei*, 

 who adopts Potonie's position, prefers to 

 •draw the comparison with liverworts, 

 which show a similar sympodial develop- 

 ment of a dichotomous branch system.* 



It seems not improbable that the condi- 

 tion 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 ap- 

 plicable to other foliar developments in 

 vascular plants, especially pteridophytes 

 such as the lycopods, equiseta and spheno- 

 phylls. 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 between widely distant groups, or 

 between the different generations of an 

 antithetic alternation. It is true that 

 Potonie prefers to regard such generations 

 as homologues, 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 dif- 

 ferent 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 slender a 

 ground as similarity of method of branch- 

 ing represented in them both. For sym- 

 podial development of- a dichotomous sys- 



* ' Beitrage z. Morph. d. Sporopliylle u. d. 

 Trophopliylls,' Hamburg, 1902. 



tem (and this is all that such 'overtop- 

 ping' actually is) has occurred in cases 

 Avhere it can not be held to have resulted 

 in a branching which is foliar; and of 

 this instances can be found without go- 

 ing so far afield as the Fucaces. If this 

 be so, then little value need be attached to 

 the comparison of such branchings in 

 plants not nearly allied to one another; 

 these may be held to be quite distinct ex- 

 amples of a general phenomenon, without 

 the one being in any sense the prototype of 

 the other. Such reflections as these indi- 

 cate that the comparison in mode of branch- 

 ing between the leaves of ferns and the 

 thallus of fucoids, which forms the ground- 

 work of the view of Potonie (or between 

 the ferns and the thalloid liverworts, as 

 may be preferred by others), are not to be 

 held as more than distant analogies; con- 

 sequently they are no demonstration of the 

 origin of the leaf by a process of 'over- 

 topping. ' 



There remains the third view, which, 

 however, is no new one ; for there have not 

 been wanting those who have assigned a 

 more prominent place to the axis in the 

 initial differentiation of the shoot. Per- 

 haps the most explicit statement on this 

 point is that by Alexander Braun, who re- 

 marks in his 'Rejuvenescence in Nature' 

 (English edition, p. 107), referring to 

 phytonic theories, that 'all these attempts 

 to compose the plant of leaves ai-e wrecked 

 upon the fact of the existence of the stem 

 as an original, independent and connected 

 structure, the more or less distinct articu- 

 lation of which certainly depends upon the 

 leaf formation, but the first formation of 

 which precedes that of the leaves.' Unger 

 also in his botanical letters to a friend (No. 

 VIII.), described how 'The first endeavor 

 is directed towards the building up with 

 cell-elements of an axis' — 'those variously 

 formed supplementary organs which are 

 termed leaves originate laterally upon it' 



