XXXIII] 



HABIT 



295 



Such vein-fusions as are found in some Hemitelias and Alsophilas are 

 obviously consequences of that condensation: they commonly appear as 

 loops parallel to the midrib, while the rest of the venation is open. But 

 occasionally fusions exist also nearer the margin. Nevertheless the venation 

 is always readily referable to a primitive Pecopterid type (Fig. 558). 



Fig. 557. Cyathca simtata Hook. A, shows the habit, with simple leaves, unique in 

 the genus. B, part of a fertile leaf. C, part of a fertile leaf more highly magnified, 

 showing the venation and sori. (After Hooker, from Engler and Prantl.) 



The surfaces of the stem and of the leaf-bases are covered by a hard 

 sclerotic coating that would be impervious for gas-interchange, were it not 

 for pneumatodes which appear on stems and leaf-bases as oval areas filled 

 with spongy tissue, as in Alsophila crinita (Vol. I, Fig. 193). On the leaf- 

 stalks they appear as the usual pale lateral lines. But the most characteristic 

 surface-features are the broad chaffy scales which form a dense covering over 

 leaf and stem while young. They are present in all the three genera, and 

 form a distinctive feature of the Family. But A. pubescens Baker, is described 



