﻿Polypodium marginellum and its immediate allies* 



Among the numerous groups of species comprising the section 

 Eupolypodium of the genus Polypodium, as represented in tropical 

 America, there are few if any which have a general structure so 

 simple and unusual as that exhibited by Polypodium marginellum 

 and several closely related species. This group, of which there are 

 at least five American members, is represented also in Africa, as 

 discussed below. The several species, which apparently are all 

 epiphytes of moist mountain forests, are closely similar in general 

 form, the fronds being of a narrowly linear type, simple, 5-25 

 cm. long, pinnately veined, and having the entire or slightly un- 

 dulate margin bordered with a capillary or flattish, lustrous, dark 

 brown or ebeneous band of sclerotic tissue, which from its firm 

 structure is not readily perishable and may even persist long after 

 the green tissue of the frond has disintegrated and disappeared. 

 This sclerotic band has, so far as can be observed, no connection 

 whatever with the fibrovascular conducting system; that is, the 

 midvein and veins, the latter terminating at a point relatively 

 remote from the margin. It seems rather to function primarily as 

 a mechanical device to give strength and rigidity to the fronds, 

 though not improbably it meets also some physiological need that 

 is not at once apparent. 



The marginal sclerotic structure just described is apparently 

 unique in Eupolypodium. What appears at first to be a similar 

 case is seen in Polypodium gramineum SwcU-tz, a West Indian plant 

 with small, very narrow, simple, entire, darkish-margined fronds; 

 but in this species the dusky border is actually composed of con- 

 ducting tissue, consisting of a marginal vein which connects the 

 excurrent ends of the once or twice-branched lateral veins. Just 

 outside of the marginal vein may be observed a delicate line of 

 greenish tissue (as is not the case in P. marginellum and its allies), 



* Published by permission of the Secretary of the Smithsonian 



