CALLOSE SUBSTANCE IN SIEVE ELEMENTS

The secondary phloem of 6 species of woody dicotyledons was examined for the occurrence of callose on the sieve plates of active sieve elements. Fluorescence and bright-field staining methods were used to detect callose. Tissue from the 6 species was killed and fixed in each of 5 solutions. Some tissue of each species was submerged in the killing solutions as quickly as possible, the remainder within 15 min after removal from the tree. In each species, some active sieve elements of the quick-killed tissue gave negative callose reactions. All active sieve elements of the delay-killed tissue gave positive callose reactions. These and other results suggest that the active sieve elements in the secondary phloem of the species studied normally lack callose and that the extent of callose deposition in these cells depended primarily upon the rapidity with which the sieve-element protoplasts were killed after wounding of the phloem. In addition, bright-field observations of sieve plates of large numbers of sieve elements from a seasonal collection of Tilia americana secondary phloem suggest that the active sieve elements normally lack callose during the growing season and that the inactive sieve elements normally possess it (dormancy callose).