Aug. s, 1918 Anatomy of the Potato Plant 227 
in the outer comers (PI. 43, C). These five groups are distinct traces, and 
their origin must be established and their course followed. A series of cross 
sections through as many intemodes as are traversed by the longest trace 
is required for this task. Since the scheme of phyllotaxis has a divergence 
of 5/13, it may be supposed that the greatest extent of any of these traces 
would not exceed three intemodes; this is actually the case. 
As stated above, there are three large and three small stem bundles 
in the intemodal region (PI. 27, A). Below the node (base of diagram, 
figure 1, which illustrates a right spiral), where the stem begins to lose 
its three-cornered appearance and takes on the quadrangular outline, 
main group A begins to widen and breaks up into two more or less equal 
portions, A 1 and A 2 . A little higher up, main group B also begins to 
widen. Immediately below the node, A 2 gives off a small bundle, a, 
which soon separates from the parent group and comes to lie between 
A 1 and A 2 . (See also PI. 27, B, a.) Just at the node, groups B and 
A 2 split, each forming two groups of nearly equal size: B gives rise to 
B 1 and B 2 , and A 2 gives rise to A 3 and A 4 . B 2 and A 4 , given off by B 
and A 2 , respectively, then divide unequally once more. These four 
last-mentioned groups pass out into the petiole forming the lateral 
traces there. The small stem group b, occupying originally the position 
between A and B, also passes out, but without division, into the petiole 
where it forms the median trace of the leaf. We see then that the 
vascular tissue of the petiole is derived from two chief sources: 
(1) One of the smaller bundles of the stem, which becomes the median 
trace of the leaf. 
(2) Branches of the two large stem groups A and B, which form the 
four lateral traces. The traces of the petiolar wings are derived from 
the lateral groups B 2 and A 4 after these have become separated from 
the stem bundles. 
At the place of first branching of A, A 1 continues the original course 
of A. At the place of second branching those portions (B 1 and A 3 ) of 
B and A 2 which do not enter the petiole unite just above the insertion 
of the leaf to form a new stem bundle. Through this fusion the stem 
becomes three-cornered again. 
The stem group A supplies two lateral traces of the petiole, gives 
rise to a new stem group A 1 , and also to a new median trace, a, which 
becomes the median trace of the third leaf above; it further gives off 
some of its vascular tissue to B 1 . B also supplies two lateral traces to 
the petiole, but does not form a third bundle; on the contrary, it receives 
vascular tissue from A 2 in compensation for the loss. 
Bundle A 1 ascends for one intemode and then forks again, but B 1 
ascends unchanged for two intemodes, then forks and gives rise to the 
same number of groups as B gave off near the first node, becoming the 
B of the second node. Bundle C similarly becomes the A of the second 
