CYATHEAE 



603 



is cup-like, and may form a complete investment for the sorus while young. 

 The disposition of the sori is fundamentally as in Gleichenia, in a single 

 series on either side of the midrib of the pinnule ; but the regularity of 

 the series is liable to be disturbed according to the mode of growth of 

 the leaf. Moreover, the identity of the sorus is not always maintained, 

 fissions (or fusions) being frequently seen. 



The development of the sorus has been followed in A. atrovirens : an 

 early condition of it is shown in Fig. 334 A, in which the sporangia near 

 the apex of the receptacle have the cap-cell already formed, while in the 

 lower sporangia on either side it is not yet cut off. The succession is 

 thus basipetal, though not markedly so, since their number is not very 

 large in this species. The sporangia are from the first more robust than 

 those of Cyathea, but less so than those 

 of Gleichenia, to which they correspond 

 in the oblique annulus, and in their 

 position in the sorus : moreover, the 

 first segmentations take a middle place : 

 the parent cell of the sporangium has 

 frequently a wedge-shaped base, and 

 the first segmentation-wall cuts one of 

 its oblique lateral walls : this type is 

 thus intermediate between that of the 

 more robust Simplices and that of the 

 smaller sporangia of the Polypodiaceae. 

 In point of its convex form and actual 

 size the sporangium of A. atrovirens 

 does not differ widely from that of 



Gleichenia dichotoma, but the stomium Too?' r=VSTrf ^S^Br' 

 is lateral as against the median dehi- S; re < a transverse section of a sporangial 

 scence of Gleichenia : still it is obvious 



that the cells of the stomium form part of the very regular series of the 

 annulus (Fig. 334 c). The regularity of the orientation is according to the 

 type of Gleichenia in the basal part of the sorus, but it is not strictly 

 maintained by the sporangia at the apex of the receptacle : in this also 

 Alsopliila corresponds to Gl. dichotoma. The typical number of spores 

 per sporangium appears to be 64. 



It is thus seen that Alsophila shows certain points of interest for 

 comparison with Gleichenia : notwithstanding the difference of habit it 

 corresponds in the position of the sorus and in the absence of the indusium, 

 while the number of the sporangia in the sorus in A. atrovirens is not 

 greatly in excess of that in Gleichenia dichotoma. There is also some 

 similarity in the form of the sporangium with its oblique annulus, and in the 

 orientation of the sporangia, at least in the lower part of the sorus, though 

 in both there is irregularity towards the apex of the receptacle. But there are 

 important differences in the position of dehiscence and in the number 



FIG. 334. 



A A Isophila atrovirens^ Prest. A young 

 sorus cut vertically, and showing a slight indica- 

 tion of basipetal succession of the sporan 



celsa, 

 sporangial 



