XII] 



MARGINAL SORUS 



210 



readily seen in Thyrsopteris (Fig. 205). Here the fertile segments being very 

 narrow the receptacle itself is almost cylindrical, as it is in Loxsoma, and in 

 the HymenopJiyllaceae. 



The marginal position of the sorus is sometimes maintained till maturity, 

 as in the Ferns just named. But in most cases where the origin of the sorus 

 is marginal, as the development proceeds it may be more or less directed 

 by unequal growth towards the lower (abaxial) surface. This is seen in the 

 " monangial sori " of the Schizaeaceae. It has been demonstrated fully for 



Fig. 212. Dicksonia Scheidei, Baker. ^ = section through a young sorus perpendicular to the 

 leaf-surface; i, ? = indusium; w^cell of marginal series. j9 = section of sorus parallel to the 

 leaf-surface, as along a line, /, i, in Fig. A, showing the receptacle bearing sporangia, s, s. 

 C= a similar section bearing older sporangia. Z)= transverse section of a young sorus showing 

 the two lips of the indusium (ind.), and receptacle between them, as along a plane, y, y, in 

 Fig. A. A section of the receptacle in the plane, x, x, in Fig. A, is superposed on the lower 

 indusial lip. The central figure shows sporangial stalks cut transversely. A—Dx2oo. 

 E, F, G, H, sporangia of Dicksonia Menziesii from four different aspects, x 50. 



Schizaea riipestris (Fig. 213), and the results become very evident in Mohria, 

 where the sporangium appears as though superficial after the first stages 

 are past, owing to a secondary development of an indusial flap at its base, 

 forming a false margin (Fig. 214). A like diversion to the lower surface as 

 the part matures— but here it is of the whole sorus — is seen in Dicksonia 

 {Cibotiuin) (Fig. 212). Such ontogenetic changes of position are common. 

 They may be seen to lead in Ferns of the Dicksonioid-Davallioid affinity 

 to a " phyletic slide " of the sorus to a definite initial position on the lower 

 surface of the leaf, as the following examples taken from closely related 

 Ferns will show^. In the first place Conard has found that the origin of 



1 As to this relationship, see Studies, vn, Ann. of Bot. 1918, p. 50, in which the conclusion of 

 Prantl is adopted, Arb. K. Bot. Gart. Breslau, i, i, p. 16. 



