14 Development of the Fern Leaf 



ning of subdivision of the secondary segments into tertiary 

 segments is shown. In Fig. i, the beginning of the incisions 

 between the primary branches of the segment's midvein, by means 

 of which the secondary segments are to be formed, is in some 

 places not evident, and in others indicated by barely perceptible 

 indentations of the leaf's margin. In Fig. 2 it is marked, and 

 in Fig. 3 these incisions are pronounced. 



In Fig. 4 some of these incisions have almost, and others 

 quite, reached the midvein, so that the secondary segments are 



FIG. 4. 



practically distinct, and the incisions, by means of which the 

 secondary segments are to be divided into tertiary segments, 

 have begun to form between the primary branches of the second- 

 ary segments' midveins. 



In Fig. i the primary branches of the segment's midvein 

 vary in degree of complexity. Those close to the apex of the 



