1 6 Development of the Fern Leaf 



ment that is to contain the latter midvein is fully formed, i.e., 

 distinct, remain simple until this segment becomes distinct. 

 There are also cases in which all the primary branches of each 

 midvein and incipient midvein of a leaf are simple when first 

 produced, and incisions, forming distinct segments, occur between 

 them coincidently with their production: in these cases the seg- 

 ments appear to be made up of lobes containing each a simple 

 vein. It is a question, however, if in the latter cases the vena- 

 tion can be called pinnate. It approaches closely the free fla- 

 bellate type. 



The depth to which the incisions between the primary branches 

 of the midvein or midveins are carried in any part of a leaf 

 determines whether the segments formed by them shall be partly 

 formed or distinct, and if distinct whether adnate or not adnate 

 to the rachis. For example, in Fig. 4 the incisions between the 

 secondary segments are carried only far enough to render these 

 segments practically distinct and adnate to the primary seg- 

 ment's midvein, which now constitutes a rachis. If these in- 

 cisions had been extended, at the points where they cease in 

 Fig. 4, along the edges of the rachis as far as the midveins of 

 the secondary segments, these segments would have been sessile 

 instead of adnate to the rachis. If the latter midveins had been 

 lengthened at base, or the incisions had extended a short way up- 

 ward beside them, these sessile segments would have been ren- 

 dered stalked. 



I have found that neither the primary branches of the mid- 

 veins of a species' leaf nor the segments formed by incisions be- 

 tween these branches appear uniformly either opposite or alternate 

 in all the leaves of any species with pinnate free venation that I 



