PTEROPSIDA 143 



The leaves, in most species, are leathery in texture, but 

 those of Leptopteris hymenophylloides are comparable with 

 those of the Hymenophyllaceae ('filmy ferns') and have a 

 thin pellucid lamina, only two or three cells thick, from 

 which stomata are completely lacking. During their develop- 

 ment the leaves of all species exhibit circinate vernation and 

 are covered with hairs. The base of the petiole is broad and 

 winged in a manner reminiscent of the Eusporangiatae and, 

 after the frond has been shed, the leaf base is persistent, 

 adding considerably to the diameter and the mechanical 

 strength of the stem. 



The fronds of Osmunda regalis are twice pinnate, those 

 produced first in each season being sterile. These are followed 

 by partially fertile fronds (Fig. 21 A), while the last to be 

 produced are often completely fertile. The fertile pinnules 

 are very reduced tassel-Hke structures, representing just the 

 midrib. In the absence of a lamina, the sporangia cannot be 

 'superficial' and are usually described as 'marginal'. In 

 partially fertile fronds of O. regalis, the fertile pinnules 

 occupy the distal regions, but in those of O. Claytoniana 

 they occupy the middle regions. Todea bar bar a has once- 

 pinnate fronds in which the fertile pinnules show scarcely 

 any modification and the sporangia are superficial, being 

 densely scattered over the under-surface of the lamina. 

 They occupy the basal regions of partially fertile fronds 

 (Fig. 21H). The fronds of Leptopteris hymenophylloides are 

 large and many times pinnate, with the sporangia scattered 

 sparsely along the veinlets of unmodified pinnules (Fig. 

 2 1 F). In no case is there any tendency for the sporangia to 

 become aggregated into sori, nor is there any sign of an 

 indusium. 



The sporangium is not strictly leptosporangiate, for 

 several cells play a part in its initiation and, at maturity, it 

 is relatively large and massive with a stout short stalk. 

 There is some variation in the shape of the archesporial cell, 

 as illustrated in Figs. 21I and 21 J, for it may be tetrahedral, 



