XII] 



THE PHYLETIC SLIDE 



223 



adaxial and abaxial flaps of the indusium appear as superficial growths back 

 from the margin (Fig. 218, A, B). But in the mature state the whole sorus 

 appears bent round to the lower surface (Fig. 218, D), though both of the 

 indusial flaps are still retained. In Histiopteris incisa varying intermediate 

 conditions are found between a marginal and a superficial origin of the 

 receptacle {R), while the lower (abaxial) indusium is not represented, and the 

 upper (adaxial) flap takes in some degree the position and character of a 

 leaf-margin (^Fig. 219). The next step, that of the actually superficial origin of 

 the receptacle, is seen in Pteris itself {P. sernilata, L. fil.); here the marginal 

 segmentation of the young leaf leads directly to the formation of the adaxial 



Fig. 2 1 8. A, B, vertical sections through the margin of the pinnule of Pteridinm aquilinuni, showing 

 how the receptacle ( x ) originates directly from the marginal segmentation, while the indusial flaps 

 are of superficial origin. ( x i8o.) C= vertical section of the fusion-sorus of Pteridiutn following 

 the line of the vascular commissure. ( x 90.) /) = section of a more mature sorus than in A or B, 

 but in a similar plane showing the mixed character. ( x 90.) 



flap, which serves as an apparent continuation of the expanded leaf-surface; 

 the sporangia appear to arise from the lower surface of the leaf, without any 

 definitely convex receptacle (Fig. 219). The steps of change in developmental 

 origin of the sorus thus seen in these three Ferns indicate a transference of 

 origin of the receptacle from the margin to the lower surface of the leaf, and 

 the change is demonstrated in Ferns so closely allied that until recently 

 they were all included in the old and comprehensive genus Pteris. A like 

 change has also been indicated by Christ, and by von Goebel for the Daval- 

 lieae (Christ, Farnkrduter, pp. 2, 287, etc. ; von Go&he\,Organograp/ne, II. Aufl. 

 p. 1140). Thus it appears to have occurred repeatedly in the Descent of 



