Development of the Fern Leaf 15 



segment are simple ; those below, by lengthening and sending 

 out branches, have begun to develop into the midveins that 

 they finally become in the secondary segments, although these 

 segments are not as yet formed. In Fig. i a this development 

 of a primary branch into a midvein has but just begun: the 

 primary branch may be described either as once forked or as an 

 incipient midvein not developed beyond its two first (basal) 

 primary branches, which are simple. In Fig. i b this develop- 

 ment has been carried a step farther, in Fig. 2 c another step, and 

 in other segments of Fig. 2, in some of Fig. 3, and in the second- 

 ary segments of Fig. 4, successively still farther. In Figs, i 

 to 3 may be seen the early stages of the similar development of 

 the primary branches of the newly-formed midveins into the 

 midveins they are to become in the tertiary segments. 



The development of the primary branch of one midvein 

 into another midvein with primary branches is brought about, 

 as may be seen from the above examples, by the primary branch 

 repeatedly lengthening and sending out successive primary 

 branches of its own on both sides. This development may 

 begin before, during, or after the formation of the incision or 

 incisions by means of which the segment that is to contain the 

 new midvein is formed. At the same time the branches pro- 

 duced by the latter midvein may or may not begin to develop like- 

 wise. Within reasonable limits, there are apparently no bounds 

 to the degree of complexity the primary branches of a midvein 

 may attain before or during the formation of the incisions that 

 set them apart to constitute midveins. On the other hand, the 

 branches produced by a midvein's primary branch during its 

 development into a midvein may, if appearing before the seg- 



