PROLIFERATING SIEVE ELEMENTS PRESENT IN BUD PHLOEM ANASTOMOSES CONNECT SIEVE TUBES OF AXILLARY BUD TRACES TO STELAR VASCULAR BUNDLES IN THEAQUATIC MONOCOTYLEDON POTAMOGETON NATANS L. (POTAMOGETONACEAE)

The stem of Potamogeton natans is characterized by a central stelar va scular system with reduced xylem and abundant phloem. Wide sieve tubes composed of short sieve-tube members joined by simple sieve plates an d associated with companion cells establish an effective conduit for a ssimilates. At each node the phloem forms a network of parallel sieve elements connecting the stem phloem to leaf and bud traces. In P. nata ns an axillary bud rarely develops into a side branch, its procambial vascular bundles are each connected to the nodal complex via separate anastomoses. Their most unusual components are the anastomosal sieve e lements (ANSE), characterized by thin cell walls pitted all over by ti ny callose-lined pores resembling plasmodesmata, which can be detected as bright areas by fluorescence microscopy after staining with anilin e blue. Several layers of ANSE make up the centre of an anastomosis an d link to both the nodal and bud stelar sieve tubes via mediating (MSE ) and connecting sieve elements (CSE). The ultrastructural differentia tion of ANSE, MSE, and CSE corresponds to that of normal sieve element s, i.e., in the mature stage they are enucleate, evacuolate, and have lost most of their cytoplasm. Their plastids are of form-P2c, containi ng many cuneate protein crystals, typical of monocotyledonous sieve el ements. Quantitative aspects of the ''pore'' areas are discussed in re lation to the functional significance of bud anastomoses.