102 Silvery Spleen wort 



often becomes an incipient or more developed midvein with 

 simple branches (PL XXV, Fig. 2). 



It will be seen that in one sense the last step is niere repetition 

 of what has gone before, and that it accords with this fern's 

 disposition to lengthen and point its leaf-segments as they de- 

 velop, to enlarge teeth into segments, and to develop branches 

 of midveins into midveins with branches. The leaves with 

 the highly developed pinnae are sometimes sterile, sometimes 

 fertile. While less common than the leaves that do not repre- 

 sent the height of the leaf -development, they are to be looked for 

 in any luxuriant mature plant, and are likely to occur on the 

 same plant with less well-developed leaves as it advances in 

 development: leaves also are likely to occur in which some of 

 the pinnae's segments are highly developed and others not. 

 There is apparently nothing here to denote subspecific variation, 

 or even monstrous development, or to entitle either the markedly 

 highly developed leaves or markedly less well developed but 

 mature leaves to a distinctive name. Yet the highly developed 

 leaves have received at least two such names, and in one instance 

 the less well developed have been interpreted as a variety of the 

 highly developed.* 



In this fern each sorus is borne on a vein which stands, or on 



* "Var. serratum" Lawson is apparently based upon the highly developed leaves. 

 SeeCanad. Nat. 181. 1864. "Athyrium acrostichoides thelypteroides" Gilbert is based 

 upon the less well developed leaves. See Fern Bulletin, 8:9-10, 1900. In his "List of 

 North American Pteridophyta," 15. 1901, Mr. Gilbert apparently reverses this arrange- 

 ment, calling the highly developed leaves the variety or " form," to which he gives the 

 name "A. thelypteroides f. acrostichoides." 



Swartz appears to have drawn his original definition of his A . acrostichoides from the 

 highly developed leaves, judging from p. 82 of his Synopsis Filicum, where this definition 

 is quoted and appears side by side with a short definition of Michaux's A . thelypteroides, 

 which Swartz evidently considered distinct. The two definitions point respectively to the 

 highly developed and the less well developed leaves of this fern. 



