302 



FILICALES 



[CH. 



differences in leaf-form are the expression of a physiological 

 division of labour connected with an epiphytic existence. Some 

 tropical species of Polypodium (sect. Drynaria), e.g. P. querci- 

 folium (fig. 234 and fig. 231, D), produce two distinct types of 

 leaf, the large green fronds, concerned with the assimilation of 

 carbon and spore-production, being in sharp contrast to the small 



Fio. 233. Poly-podium Billardieri Br. (J cat. size.) Middle Island, New 

 Zealand. From specimens in the Cambridge Herbarium. 



slightly lobed brown leaves which act as stiff brackets (fig. 

 234, M) for collecting humus from which the roots absorb raw 

 material. Similarly in Platycerimn the orbicular mantle-leaves 

 differ widely from the long pendulous or erect fronds fashioned 

 like the spreading antlers of an elk. In Hemitelia capensis, a 

 South African Cyatheaceous species, the basal pinnae assume 



