Development of the Fern Leaf 2 1 



the other (d) of these two segments has begun to subdivide into 

 segments. 



Since, when a simple leaf or segment of a leaf with free fla- 

 bellate venation subdivides into segments, the incision occurs be- 

 tween the two vein-branches formed by the fork of the vein that 

 enters the simple leaf or segment, and since each of these branches 

 thus becomes the vein entering one of the two new segments, this 

 branch's starting-point — namely, the apex (point of forking) of 

 the vein that entered the original simple leaf or segment — may 

 be considered the starting-point or base of the segment this 

 branch enters.* Any two segments formed by the subdividing 

 of a simple leaf or simple segment of a leaf with free flabellate 

 venation are thus situated at the apex (point of forking) of the 

 vein that entered the original simple leaf or segment. Hence, if 

 the part of the original simple leaf or segment that contains this 

 vein becomes sufficiently attenuate it wUl constitute a stalk bear- 

 ing two segments at apex. If, then, either one of the latter seg- 

 ments, which may be designated as a, becomes transformed in 

 like manner into a stalk with two segments at apex, and the 

 other, which may be designated as b, remains undivided, the 

 latter stalk wUl have the same starting-point as b, since it is 

 formed of the lower part of a, and since a and b had the same 

 starting-points. This stalk wUl, therefore, separate the pair of 

 segments at its apex from b by its length, which is the length of 

 the vein occupying it, namely, the vein that entered a. This 

 stalk wUl thus constitute a rachis. If b was the left one of the 



* The end of the incision by means of which the two segments are formed cannot 

 be held to determine the starting-points of these segments, for it varies in position in 

 different stages of the segments' development. 



