14 Development of the Fern Leaf 
ning of subdivision of the secondary segments into tertiary 
segments is shown. In Fig. 1, 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 

Fic. 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. 1 the primary branches of the segment’s midvein 
vary in degree of complexity. Those close to the apex of the 
