510 FILICALES 
and Angiopieris (Fig. 278 A and c): or they may be more extended along 
the veins, as in Archangiopteris (Fig. 278 8): or they may occupy the 
whole space from margin to midrib, as in Danaea (Fig. 278 £). But in 
all these cases the disposition of the sorus is essentially the same, and 
the differences those of detail only. In Aazl/ussta, however, the sori are 
dotted over the broad under-surface without apparent order, a condition 
Fic. 279. 
Vertical longitudinal section of the stem of a young plant of Azgiopteris evecta. b=the 
youngest leaves still quite covered up by the stipules, 2é; s¢=stalk of an unfolded leaf 
with its stipula, 24; 2, everywhere the leaf-scars on the basal portions_4/, from which the 
leaf-stalks have separated ; ¢, c, the commissures of the stipules in longitudinal section ; 
w,w, the roots. Natural size. (After Sachs, from Goebel’s Outlines.) 
which appears widely different from the rest (Fig. 278 D): but comparison 
of leaves of Danaea, and especially of those which are only partially 
fertile, gives the clue to an explanation; for there the normally elongated 
sori are found to show occasional fissions, and the partial sori, with 
circular outline like those of Kaz/fussia, appear isolated upon the enlarged 
leaf-surface (Fig. 281 a, 4, c). It seems probable that the condition of 
Kaulfussta was acquired during descent in some such way as this, on the 
gradually broadening leaf. The frequent occurrence of sori showing similar 
