PTEROPSIDA 163 



greatly expanded (Fig. 25J), so that, in spite of its marginal 

 origin, the sorus appears to be superficial at maturity. This 

 represents an early stage in the evolutionary process which 

 Bower called the *Phyletic Slide', whereby the sorus ulti- 

 mately has a superficial origin despite is marginal ancestry. 



Davallioideae 



Davallia, likewise, has a superficial sorus at maturity, 

 covered by a funnel-shaped indusium, but which, neverthe- 

 less, is marginal in origin. The stem is creeping, with a peculiar 

 type of dissected solenostele, and is clothed with scales. 



Oleandroideae 



Nephrolepis has upright, dictyostelic stems with long run- 

 ners, by means of which vegetative reproduction occurs, for 

 the tips of the runners are capable of rooting and turning 

 into normal erect stems. Within the genus, there is a wide 

 range of soral form. A'^. davallioides (Fig. 25N) is very 

 similar to Microlepia, in that the upper indusium is scarcely 

 larger than the lower. In A'^. acuta, the sorus is superficial, 

 not only at maturity, but also in origin. A'^. dicksonioides 

 shows a different evolutionary trend, in that adjacent sori 

 are sometimes fused, and this trend has proceeded so far in 

 A^. acutifolia that the margin of the pinna has a sorus run- 

 ning continuously along it, between two linear indusia. 



Pteridoideae 



It is generally accepted that the sorus of Pteridium evolved 

 in a similar way to that of Nephrolepis acutifolia, for it, too, 

 is continuous along the margin of the pinnule (Fig. 25Q) 

 and is protected by two indusia. The upper indusium (i) is 

 relatively thick, but the lower one (2) is thin and papery. 

 Pteridium is one of the most successful ferns in its ability to 

 compete with flowering plants and this may, to some extent, 

 be due to the great depth at which its rhizomes spread be- 

 neath the surface of the soil. Its stele is a dicycHc perforated 



