74 Maidenhair Spleenwort 



of the leaf containing the section of primary midvein between 

 that pair of primary branches and the pair next above becomes a 

 section of rachis. The primary midvein thus becomes merged 

 in a rachis, remaining visible as a midvein in the leaf's apical section 

 only. While pinnae and primary branches of this midvein are 

 separating from the base of the apical section, this section and this 

 midvein lengthen at apex, the midvein sending out additional 

 primary branches, but this takes place slowly; the apical section, 

 therefore, is usually rather short. One or more pinnae are usually 

 to be seen in the process of forming on and of separating from it. 



In the very young leaf the pinnae newly formed are often, like 

 the leaf-blade newly formed, obcordate, with their midveins not 

 developed beyond two simple primary branches. Such pinnae 

 resemble the pinnae of Asplenium Glutei, Gilbert.* The various 

 forms of the early pinnae can be seen from Plate XV. As the leaf 

 develops, the pinnae lengthen and broaden, gradually assuming 

 their mature forms, their midveins lengthen and send out addi- 

 tional primary branches, and these midveins' primary branches 

 mostly become more or less complex. 



The pinnae are usually crenately toothed or lobed, but, occa- 

 sionally, incised. The so-called "var. incisum" is based upon 

 monstrously developed sterile leaves with deeply incised pinnae. 



* See Fern Bulletin, 8: 62-63. 1900. 



