14 HYPOLEPIS, ETC. [CH. XXXVI 



Transition from Dicksonioid Ferns to tfie Davallioids 

 AND Pteroids 



The argument as to the importance of the marginal position of the sorus 

 has been advanced in Vol. ll, Chapter XXVI, and it has been seen how that 

 position, so pronounced in the Schizaeaceae, is present also in the Hymeno- 

 phyllaceae, Loxsomaceae, and Dicksoniaceae, while in the Dennstaedtiinae 

 (Vol. II, Chapter XXX), and particularly in Hypolepis (Vol. Ill, Chapter XXXVl), 

 it may be departed from by gradual steps, which run parallel with other signs 

 of advance in organisation, anatomical as well as soral. These indications 

 naturally prepare the way for the study of other Ferns either with the 

 marginal position fully maintained, or gradually departed from, as in 

 Dennstaedtia and Hypolepis. It will be seen that such changes are linked with 

 others that lead to a condition characteristic of the more advanced Lepto- 

 sporangiate Ferns. It is believed that such types are actually seen in the 

 Davallioid and Pteroid Ferns, the discussion of which will be taken up from 

 the point of view thus indicated. 



These two groups have usually been treated apart. Diels places the 

 Davallieae quite separate from the Pterideae {Nat. Pflanzenfam. I, 4, pp. 204, 

 254). The diagnoses are definitely systematic rather than comparative. The 

 aim seems to be to distinguish between these two groups rather than to 

 detect similarities, and so to arrive at diagnostic rather than at morphological 

 ends. But if an evolutionary view be entertained, the fundamental idea being 

 the origin of both of these groups of Ferns from some source such as the 

 Dicksoniaceae taken in its widest sense, and if the possible progressions in 

 respect of various criteria of comparison be examined, the probability will 

 be seen to emerge that the two are really related, and that they mutually 

 throw light upon one another. The leading diagnostic feature is that in the 

 Davallioid Ferns the sori are usually — but not always— separate and marginal, 

 with a double indusium: but in the Pteroid Ferns they are merged into 

 linear sequences, and tend to spread on to, or even over, the lower surface, 

 while the lower indusium becomes vestigial or abortive. It will be seen, 

 through intermediate steps, how these states were attained. The Davallioid 

 Ferns will be discussed first because they retain more persistently the individual 

 marginal, two-lipped sorus, which is believed to have been primitive, as seen 

 in the Dicksoniaceae. 



BIBLIOGRAPHY FOR CHAPTER XXXVI 



587. Mettenius. Farngattungen, v, Cheilanthes. 1859. 



588. Hooker. Synopsis Filicum, p. 128. 1873. 



589. DiELS. Natiirl. Pflanzenfam. i, 4, p. 277, etc. 1902. 



590. Gwynne-Vaughan. Ann. of Bot. xvii. p. 694. 1903. 



591. Carse. Trans. New Zealand Inst. XLVii, p. 85. 



592. Bower. Studies VII, Ann. of Bot. xxxii. p. 50. 1918. 



