Development of the Fern Leaf 23 



stalks will be the apex of the former stalk, and the three stalks 

 will thus constitute a dichotomous rachis. 



It is thus apparent that each section of rachis in a leaf with 

 free flabellate venation consists of the part of some segment sub- 

 divided into segments that contained the vein entering the seg- 

 ment, and which has become attenuated. This attenuation of 

 this part of the segment that becomes the section of rachis may 

 take place before, during, or after the subdividing of the segment 

 into segments. If before, the segment, until it subdivides into 

 segments, will appear merely stalked, and if it be one of the two 

 terminating a rachis or petiole of a leaf, will appear raised above 

 the other by its stalk. 



It is also apparent that whether the rachises of the leaf with 

 free flabellate venation are simple, forked, or otherwise com- 

 pound, and whether they bear segments on one or both sides, as 

 well as at apex, or at apex only, depends upon which of the leaf's 

 segments subdivide into segments and become attenuate below 

 these segments in the course of the leaf's development. 



There are certain cases in which the compound or partly 

 compound leaf with free flabellate venation may resemble or be 

 indistinguishable from that with free pinnate venation. For 

 example : 



(i) When in the flabellate-veined leaf segments are borne on 

 both sides of the rachises, and attenuation of the parts of segments 

 of which the rachises are composed has not been very complete, 

 so that a small amount of leafy tissue forming a wing remains 

 about the vein each of these parts contains, this leaf may re- 

 semble or may be indistinguishable from the pinnate-veined leaf 

 in which the incisions between the primary branches of the mid- 



