ABERRANT SIEVE ELEMENT DIFFERENTIATION IN PRIMARY LEAVES OF VIGNA-SINENSIS ENDL AFFECTED BY COLCHICINE

In primary leaves of Vigna sinensis Endl. seedlings growing in the presence of colchicine, some giant polyploid cells at the phloem pole of the vascular bundles differentiate into an extraordinary cell type acquiring most of the prominent features of the sieve elements. These aberrant sieve elements (a-SEs) show some exceptional features not found in normal sieve-tube elements. A heavy secondary thickening is deposited on their walls, which in surface view exhibits a reticulate or 'pitted' appearance. The thickenings reach the same depth into the cytoplasm and show a diffuse orientation of cellulose microfibrils. Amorphous callose is deposited around some fine cytoplasmic strands penetrating the secondary wall up to the primary one along the longitudinal and particularly the transverse walls. In the latter walls sieve pores are not formed. At first, the a-SEs undergo a partial protoplasmic degradation like normal sieve-tube elements but finally autolyse completely. During this process, the major portion of the unmasked primary and secondary wall is dissolved. The observations show that (a) a vascular bundle initiation and development proceeds in the affected primary leaves, in the absence of cell divisions. 'Vascular bundles' are formed consisting of a single row of cells, in which a-SE differentiation dominates. (b) The 'sieve element differentiation' is initiated without differential divisions. (c) Although the microtubules are absent, the cells determined to become a-SEs activate another morphogenetic mechanism to control a more or less patterned deposition of a secondary wall layer.