The Sources of Sugars in Sieve-tube Sap

All the leaves of rooted willow cuttings were enclosed in aglass chamber for 2 hours, during which time they assimilated20 µ. of ¹⁴CO₂. A number of groups of aphids were situatedalong 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 rosesharply to a peak, then fell, steeply at first, then more gradually. Twenty-four hours after ¹⁴CO₂ assimilation, substantial activitywas still evident in the honeydew. In the case of other groupsfarther from the leaves, the maximum activity reached was notso high, while those aphids with a number of other groups ofaphids between them and the leaves excreted very little ¹⁴C.They did, however, excrete sugar at about the same rate as theother group, indicating that the stem reserves compensated forthe decrease (due to the other feeding aphids) in sugars directfrom the leaves. In one instance a mean rate of translocationof 7.5 cm./hr. was obtained.