50 Thompson—The Structure and Development of 
clearly distinguishable. In the bending out of the leaf bundles 
into the petioles, the same crossing of the internal phloem to 
the exterior takes place in the leaves of the seedling, that has 
been described above for adult leaves. Scott in his work upon 
Ipomea versicolor found that in the hypocotyl near its junction 
with the root, the internal phloem passed through the xylem 
and joined the external phloem. He was thus able to prove 
the continuity of the phloem throughout the plant. Similar 
phenomena were observed by Gerard in different plants. There 
is no trace of any continuity between the external and internal 
phloem of the hypocotyl of Gelsemium. The course of the 
bundles throughout the hypocotyl and stem is indicated in the 
diagrammatic figure below. 
o, origin of first pair of internal phloem patches; o!, origin of second pair; co, 
cotyledon; /, first foliage leaf; 7*, second foliage leaf; 73, third foliage leaf; ga, grow- 
ing apex; x, crossing of internal phloem to exterior. 
