EXTERNAL CHARACTERS 631 



leaves of young Ferns, and lies at the basis of the architecture of the mature 

 leaves, finds its counterpart in the dichotomy seen in certain strobiloid 

 Pteridophytes. 



It is true that the Leptosporangiate Ferns show a very distinctive mode 

 of segmentation both of apex and margin of the leaf. But it has been 

 shown that in this character the Osmundaceae form an intermediate step 

 from them to the Marattiaceae, while the latter diverge clearly from the 

 Leptosporangiate type. It is thus seen that the definite segmentation of 

 the Leptosporangiate type is no essential character of the Fern-leaf at large. 

 Such considerations point to the justness of the view that the Fern-leaf, 

 however different in size, in continued apical growth, and in its segmentation, 

 is essentially comparable with the smaller and simpler leaves of the strobiloid 

 forms. We shall therefore accept the conclusion that in the evolution of 

 Ferns some such leaf-enlargement as is faintly indicated in certain strobiloid 

 Pteridophytes, and notably in the Ophioglossaceae, was carried out to a 

 higher degree than in any other Archegoniate Plants. It would appear 

 probable that the Ferns, developing early towards megaphylly, worked 

 out to the fullest such methods of leaf-enlargement as are outlined in 

 some other early types ; in fact, that they were ultimately derived from a 

 smaller-leaved ancestry, with a strobiloid shoot not unlike that which 

 remained in the rest persistently small-leaved. 



As regards the differentiation of their leaves, Ferns show a comparatively 

 low position. In a very large proportion, in which are included most of 

 the types which are held as primitive, the leaves are general-purposes leaves : 

 each serves at first for protection of the apical bud, and on unfolding is 

 at once an organ of assimilation and of propagation. The differentiation 

 of trophophylls and sporophylls is usually marked by a reduction of the 

 assimilating surface where the sporangia are borne : examples are seen in 

 Struthiopteris and BkcJinum, in Acrostic/utm and Platyccnnm, and the 

 distinction is to be held as a morphological advance which had, however, 

 already made its appearance in the Ferns of the Carboniferous Period. A 

 good example of this is seen in the Hymenophyllaceae, where the leaves 

 are undifferentiated in Hymenophyllitm : but in certain species of Trichonwncs 

 ( Feea.\ the genus which on other grounds is held to be more specialised 

 than Hymenophyllnm, a distinction of sporophylls from trophophylls is seen. 

 Sometimes the differentiation may be between parts of the same leaf, as 

 in Osniniida, and the fact that within this genus the relative position of the 

 sterile and fertile parts may vary indicates that the distinction is not very 

 deep-seated. Innumerable middle-forms between the sterile and fertile 

 conditions further indicate how imperfect the differentiation actually 

 is. A further specialisation of certain leaves as protective scales is seen 

 in Osmunda and in some elongated rhizomes : in such cases the rudimentary 

 leaf-apex shows that these are potentially normal leaves diverted to the 

 protective duty. It thus appears that the differentiation of the leaves in 

 Ferns is not on a high scale : that they are all essentially of one type, and 



