658 PROFESSOR F. O. BOWER ON 



in the state of impressions, has supplied important features. The second avenue 

 has, however, been somewhat neglected, and especially in the case of those plants 

 which should serve as the basis for such comparisons — the Pteridophyta. I am not 

 aware of any methodical synthesis having been given of the facts relating to the 

 juvenile leaves of the Pteridophyta, so as to make a general comparison of them 

 possible. In many of the more important primitive types the facts are still very 

 imperfectly known. The attempt has therefore been made here to supply the 

 material for pursuing this second avenue of study, and to place the results in relation 

 to those from other sources. It is only when the facts have been ascertained and 

 the underlying principles have been laid down, in some such way as this, that a 

 beginning can properly be made for the discussion of such larger questions as the 

 phyletic origin of that member designated as the " leaf," and its probable relation 

 to the axis which bears it. 



But before the detailed description of the juvenile leaves is entered upon, it 

 will be well to summarise the views which have been expressed, and the con- 

 clusions attained by others, on the subject of leaf-architecture, in so far as they have 

 been based upon the elaborate, leaves of the Filicales. Hofmeister # states that 

 " the formation of the pinnse of the frond in the species of Pteris, as in the rest of 

 the Polypodiaceae, is the result of a true bifurcation of the punctum vegetationis 

 . . . each of the new shoots is alternately more strongly developed, thus changing 

 the direction of the bifurcation to the right or to the left. The weaker one is pushed 

 on one side so as to appear to be lateral." The continual change in the direction of 

 the less vigorous bifurcations causes the feather-like form of the frond, whose seg- 

 ments (as is well known) are in no species exactly opposite to one another. This 

 description, based on a special example, includes facts which will be found to be of 

 wide application. It is doubtless this passage to which Sachs refers in his Text- 

 book,^ where he introduced the terminology which has since been generally adopted 

 for branching. He noted the apparently true dichotomy in Platycerium alcicorne, 

 and that " according to an older statement of Hofmeister it appears that branching 

 of Fern-leaves generally commences dichotomously, although mature leaves mostly 

 resemble a monopodium." Further, he notes that " since the branches or lobes are 

 apparently always alternate and not opposite, and the terminal lobes of the leaves 

 are frequently developed as equally strong bifurcations, leaves of this kind may be 

 considered, according to Hofmeister's hypothesis, as dichotomies developed in a 

 sympodial manner . . . the midrib representing the sympodium, and the apparent 

 lateral branchlets the weaker branches. ... A similar interpretation may perhaps 

 be permitted for the simply pinnate leaves of the Cycadaceae." 



The observations of Sadebeck J translated these conclusions into terms of cellular 

 construction, thus actually demonstrating the dichotomous character of the ultimate 



* Higher Cryptogamia, p. 209. t First English edition, p. 161. 



I Friedrichs-Eealschule Jahresbericht, Berlin, 1874. 



