LEAF-ARCHITECTURE AS ILLUMINATED BY A STUDY OF PTERIDOPHYTA. 659 



branchings of the lamina in the juvenile leaf of Asplenium. They were followed 

 closely, in 1875, by Kny's monograph on the Parkeriaceae. Here the demonstration 

 was given that where the single initial cell is still active in leaf-forming pinnae, 

 these do not necessarily coincide with segments from the initial cell. # In the same 

 year Prantl's monograph on the Hymenophyllaceee appeared. The general 

 conclusion from his detailed developmental and comparative study was that 

 in them the leaves, simple or complicated, were the result of purely dichotomous 

 branching, and usually developed sympodially. He suggests that this conclusion is 

 applicable also to other Ferns. t In 1881 he published similar observations on 

 the Schizaeaceae. The result was that he was able to refer the branching in all of 

 them ultimately to dichotomy. He for the first time followed carefully the steps of 

 departure to the sympodial development, both in the juvenile leaves and in the 

 pinnae of the adult leaf: and he constructed diagrams illustrating that progression 

 for Ly g odium circinatum.\ 



The term "dichotomy" has appeared repeatedly in the preceding paragraphs. 

 It must not be assumed in all cases where it has been used, or where it will be used 

 in this memoir, that there has been the equal division of an apical cell. The 

 observations for or against such a conclusion are mostly wanting. Still, a near 

 approach to such a theoretical perfection of dichotomy, if not an actual demonstra- 

 tion of it, has been given by Sadebeck in the seedling leaf of Asplenium. From 

 such a condition, which seems actually to exist in some cases, the gentlest 

 gradations lead to what is clearly monopodial branching. What is often seen is 

 then " apparent dichotomy," that is, the " continuation of one shoot by two equally 

 strong ones." § Cases where the resulting shoots are not equal, lead to what in our 

 descriptions are designated " sympodial developments of dichotomy." It is believed 

 that the fundamental construction in leaves was the true dichotomy, and that these 

 are derivatives from it along lines which are clearly suggested in the juvenile leaves 

 of many Ferns. 



It will be superfluous here to abstract the scattered literature giving details of 

 further isolated examples, whether in adult or in juvenile leaves, where the demon- 

 stration of dichotomous branching has been given. It must suffice to say that in 

 very many cases the dichotomy is obvious enough. Accordingly, we may pass 

 directly to the general statements of Goebel on the subject of leaf-construction in 

 Ferns, given in his Organographie, as representing the outlook of present morphology 

 on this matter. || He noted the similarity of juvenile leaves, even in Ferns where the 

 adult leaves are divergent in character. These may be either dichotomous or 

 sympodial in their development. The latter is the case where the growth is apical 

 (Scheitelzellwachsthum), but apical growth merges into marginal (Randzellen- 



* P. 41, Taf. vii. f P. 14- I PI. i. 



§ Lang, Address to Sec. K, Brit. Assn., 1915, p. 8. 

 || 2te Aufiage, 1913, p. 337 ; see also Engl, ed., part ii, pp. 313-321. 



