PTEROPSIDA 155 



ated with a larger number of sporangia in the sorus than is 

 usual in the genus. Yet, in this species, the dermal append- 

 ages are hairs, whereas scales are commonly present in 

 others. Divergent facts such as these serve to emphasize the 

 point that the evolution of different characters does not 

 necessarily keep step, the result being that most organisms 

 show a combination of advanced and primitive characters. 

 This is why it is unwise to focus attention unduly on one 

 character, when attempting to assess the relative advance- 

 ment of taxonomic groups. 



The sporangia, in strong contrast to those of the Schizae- 

 aceae, are borne superficially on the adaxial side of the 

 frond. They develop simultaneously and are arranged in 

 sori containing, often, only a single ring of sporangia, seated 

 either at a vein ending or, more usually, over the middle of a 

 vein. There is no indusium at all covering the sorus, whose 

 only protection is a covering of hairs or scales. Each 

 sporangium is pear-shaped (Figs. 24E-G), has a stout stalk, 

 and dehisces by means of an apical slit. Dehiscence is 

 brought about by the contraction of the thickened cells of 

 the annulus, which runs obliquely round the sporangium 

 wall. Large numbers of sporangia are liberated from each, 

 ranging from 128 to more than 1,000. 



The gametophyte is primitive, in that it is large, massive 

 and slow growing. When old, it becomes much fluted and 

 develops an endophytic mycorrhizal association. The anther- 

 idia are larger than in any other leptosporangiate fern and 

 resemble those of the Osmundales. Those of G. laevigata are 

 as much as ioo/a in diameter and contain several hundred 

 antherozoids. 



Hymenophyllaceae 



This group is commonly referred to as 'the filmy ferns', 

 because of their delicate fronds, the lamina of which is 

 usually only one cell thick. There are some 300 species of 

 Hymenophyllum, of which two occur in the British Isles, and 



