The Sources of Sugars in Sieve-tube Sap

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Abstract
All the leaves of rooted willow cuttings were enclosed in a glass chamber for 2 hours, during which time they assimilated 20 μ. of 14CO2. A number of groups of aphids were situated along the stem, their honeydew was collected hourly on filter-paper, and the specific activity of the honeydew sugars determined. The activity in the honeydew of groups nearest the leaves rose sharply to a peak, then fell, steeply at first, then more gradually. Twenty-four hours after 14CO2 assimilation, substantial activity was still evident in the honeydew. In the case of other groups farther from the leaves, the maximum activity reached was not so high, while those aphids with a number of other groups of aphids between them and the leaves excreted very little 14C. They did, however, excrete sugar at about the same rate as the other group, indicating that the stem reserves compensated for the decrease (due to the other feeding aphids) in sugars direct from the leaves. In one instance a mean rate of translocation of 7.5 cm./hr. was obtained.

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