DENNSTAEDTIINAE 599 



whole sorus, and it is on this side that the annulus may be seen to show 

 the last traces of continuity at the base. The orientation of the sporangia, 

 as described, is not always maintained in the mature state; transverse 

 sections of the mature sorus show some latitude in this respect ; but this 

 may be due merely to a twisting of the long thin stalk. This seems 

 the probable explanation, since the young sporangia show a fairly accurate 

 orientation. 



Sections of sporangia show that the number of spore-mother-cells in 

 each sporangium is variable : eight, twelve, and sixteen have been observed. 

 Countings of mature spores have shown that in certain cases the full 

 number of 64 may be produced. 



Deviations from the basipetal succession have been observed in D. 

 davallioides (Br.), Moore, and in D. rubiginosa (Kaulf), Moore. Even in D. 

 apiifolia isolated cases have been seen of a sporangium seated near the apex 

 of the receptacle, apparently arrested in its development, and representing 

 a less advanced state than those surrounding it. In D. davallioides, cases 

 of this sort are of fairly common occurrence towards the apex of the 

 rather elongated conical receptacle. These may be held to be transitional 

 forms to what is seen in the allied species D. rubiginosa, the sorus of 

 which is represented in longitudinal section in Fig. 332 bis, B. Here, upon 

 a relatively short receptacle, and between indusial flaps which are also 

 short, the sporangia are disposed with no definite succession ; the 

 persistent stalks of two old sporangia are near the apex of the receptacle, 

 and the younger sporangia below, with spores and tetrads, give a slight 

 indication of a basipetal succession, but still younger sporangia are disposed 

 irregularly among them. The sorus, though showing some slight signs 

 of a basipetal succession at first, is clearly of that type which we shall 

 designate "mixed," that is, with the sporangia produced in no definite 

 succession, but the younger interspersed irregularly among those which are 

 more advanced ; correlated with this we find the receptacle short, but 

 wide. The sporangium also shows a difference from D. apiifolia, the 

 annulus being definitely interrupted at the insertion of the stalk as is shown 

 in Fig. 332 bis, D, which represents a dehiscent sporangium seen from 

 below, (st.) indicating the insertion of the round stalk. We shall subse- 

 quently see that, by these gradations in Dennstaedtia, a transition is indicated 

 between two fundamental types of soral arrangement, the basipetal 

 succession of sporangia, and the mixed, where the various ages are irregularly 

 interspersed. Dennstaedtia is thus found to correspond to Microlepia in 

 the occasional departure from the typical basipetal succession of the 

 sporangia in the sorus : this condition leads on to that seen in the 

 genus Davallia. 



The other genera included in the Dennstaedtiinae by Prantl are 

 Leptolepia, Saccoloma, and Hypolepis. The first two of these are closely 

 related to Microlepia, as their numerous synonyms show : many of 

 their species have indeed been included in that genus. Hypolepis is 



