640 Wallace.—On the Stem-Structure of 
bundles, each occupying a comer of the five-angled stem. 
Alternating with these corner bundles and more deeply 
seated are three larger bundles. On a casual examination 
these would appear to be the only bundles present at this 
early stage, namely five outer and three inner ; but more 
careful inspection reveals that there are actually five inner 
bundles. The remaining two consist each of a few thin- 
walled, mostly narrow elements, of which those nearer the 
surface of the stem are differentiated as sieve-tubes with 
their companion-cells. 
At first these bundles consist solely of phloem; later they 
are collateral with xylem on the inside ; finally they become 
bicollateral, like the rest of the bundles. 
One of these imperfect bundles is thicker than the other 
(/ and p' in Fig. 1) and develops xylem sooner (Fig. 2). 
On approaching the node below, the stronger bundle gives off 
a branch which joins with other bundles to form one of the 
three large inner bundles of the internode next below, and 
then, diminished in thickness, proceeds downward through 
that internode of which it constitutes the thinner imperfect 
inner bundle. It seems probable that the two bundles just 
mentioned are the lower portions of leaf-traces: that the 
larger bundle seen in a transverse section is a trace of a 
lower leaf cut higher up in its course, while the smaller one 
is a trace of a higher leaf cut lower down. 
Medullary Phloem. 
Most Cucurbitaceae have bicollateral bundles : to this rule, 
however, the families Zanonieae, Fevilleae, and Gynostem- 
meae are exceptions 1 . The idea conveyed by the term 
bicollateral , invented by De Bary, is that of a bundle con¬ 
sisting of a mass of xylem between two masses of phloem, 
all the product of a single procambial meristem. Weiss 2 
1 Solereder, Syst. Anat. d. Dicotyledonen, p. 439, 1898. 
2 Scott and Brebner, £ On Internal Phloem/ Annals of Botany, vol. v, 1891, and 
works there cited. 
