626 GENERAL COMPARISON OF THE FILICALES 



Examples illustrating that this has actually occurred have already been seen 

 in the living Marattiaceae; while Angiopteris and Marat tin have upright 

 and radial stocks., that of Danaea becomes oblique or even prone as it 

 grows older, and Kaulfussia, with its longer internodes, is a creeping form. 

 In all of these, however, where the embryo is known, the shoot is in the 

 first instance erect. It seems plain that there has been a transition from 

 the upright and radial to the prone and dorsiventral type. 



In the living representatives of those sequences of Ferns which culminated 

 in the Leptosporangiate group dorsiventrality is more common, and it is 

 already seen to be prominent in such early types as the Schizaeaceae, 

 Gleicheniaceae, and Matonineae, though the Cyatheae and Dicksonieae are 

 strongly radial. There is some reason on anatomical grounds for thinking 

 that the living Hymenophyllaceae show in their radial types a recovery of 



'< 77 



FIG. 347. 



Portion of the leaf surface of a seedling of aspleniutii scrft-iitiiii, showing how 

 dichotomy accompanies the marginal growth. X 190. To the left a diagrammatic 

 representation of the same. (After Sadebeck.) 



the upright shoot from the creeping rhizome, and this may have occurred 

 in others of the Leptosporangiate Ferns. However this may be, the 

 Leptosporangiate Ferns show radial and dorsiventral development so 

 intimately intermixed that it is more difficult in them to trace the probable 

 evolutionary relations than in those groups which are clearly indicated as 

 the most ancient. But taking the facts over all, it appears reasonably 

 probable that the primitive shoots of Ferns were radial, and that dorsi- 

 ventrality was here as elsewhere derivative. 1 



In some Ferns the axis remains unbranched, as in the Marattiaceae. 

 In others dichotomous branching of the axis is seen to occur, and there 

 is reason to recognise this as a primitive mode of increase, since it occurs 

 characteristically in relatively early forms, such as in Lygodiiun^ in the 



: Mr. Tanslcy remarks very pertinently that "dorsiventrality is not very common in 

 fern steles, in spite of tin.' prevalence of creeping rhizomes" (.Viw Phytologist, 1907, p. 112). 

 To those who hold that vascular structure follows rather than dominates development this 

 is important evidence in favour of a primitively radial construction of the Fern-shoot. 



