Development of the Fern Leaf 21 
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 will 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 6, remains undivided, the 
latter stalk will have the same starting-point as ), since it is 
formed of the lower part of a, and since a and 6 had the same 
starting-points. This stalk will, therefore, separate the pair of 
segments at its apex from 6 by its length, which is the length of 
the vein occupying it, namely, the vein that entered a. This 
stalk will 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. 
