so 



PTEROID FERNS 



[CH. 



surface, is essentially of marginal origin ; meanwhile strong growth of the 

 adaxial segments produces the indusial flap (i^), which thus appears as though 

 it had been of marginal origin, but was really superficial. There is no indica- 

 tion of a lower indusium. Later, single cells from the apex of the now clearly 

 superficial receptacular ridge (R, Fig. 61$, C) grow out to form sporangia (D) ; 

 and others appearing later, with which hairs are associated, indicate a basipetal 

 succession, which is, however, not strictly maintained. Beneath the receptacle 



liowini 



Fig. 615. Sections vertically through the margins of young pinnules oi His(iopfe7is iiicisa, 

 the relation of soral origin to the marginal segmentation. A' = receptacle, .5'= sporangium, /^'=\n- 

 dusial flap. It is seen that the receptacle is close to the margin in A, B, but definitely superficial 

 \n D. ( X 200.) 



a vascular commissure appears : thus the structure comes to be that shown in 

 Fig. 616. It is concluded that the sorus of//, iiicisa is essentially marginal, 

 as in Pteridiinn and Paesia, but that it is more advanced towards a super- 

 ficial origin than either of these, and that this goes along with complete 

 abortion of the lower indusium. These observations indicate what is the 

 probable phyletic history : that H. incisa originated from some Dennstaedtioid 

 source. The steps of advance which it shows are: (i) some degree of 

 elaboration of the vascular system, (ii) a tendency towards reticulate venation, 

 (iii) the presence of dermal scales, (iv) loss of the lower indusium, (v) a 

 tendency for the sorus to slide to the lower surface: this last comes out 

 more clearly in the mature than in the young state. On all of these grounds 

 collectively we see in//, incisa a nearer approach than in other Ferns to the 

 condition seen in the genus Pteris. 



