A histochemical test for functioning sieve tubes, based on the 5-diazouracil sugar reaction, has been used to identify functioning sieve tubes in Fraxinus americana L. The plant material was lyophilized, treated with the reagent, and sectioned in a cryostat. Before the resumption of cambial activity, when buds are still closed (late April – early May), starch grains of the storage tissues in bark and wood disappear and rays, cambrium, and phloem are loaded with sugars. In early spring, sugars begin to accumulate in last year's sieve tubes, especially in those which had not been fully differentiated in the previous active season and those which were functioning but had formed toward the end of the active season. Girdling experiments, carried out during May, showed that translocation in the phloem does occur, to some extent, during the period when the buds develop and the young leaves grow. The reactivated sieve tubes provide the channel of translocation to developing buds for sugars, formed of the dissolution of starch grains. The first early-phloem sieve tubes are formed at the end of May and loaded with sugars during June. There seems to be a short period of simultaneous import into young leaves via reactivated phloem and export via new phloem. After this, last year's sieve tubes are crushed. A noticeable pressure flow does not appear in the main stem until mid-June.
Translocation of Organic Substances in Trees. III. The Removal of Sugars from the Sieve Tubes in the White Ash (Fraxinus Americana L.).