Differentiation of the Sieve Plate of Cucurbita: A Further View

A re-examination of sieve plate differentiation in Cucurbita maxima was undertaken in order to determine the relation between the development and dissolution of pore sites and the growth of the permanent part of a sieve plate. Pieces of young petioles and internodes fixed in glutaraldehyde and post-fixed in osmium were used for the study. A pore site first becomes destinguishable by a pair of flat callose collars around a plasmodesma, one on either side of the sieve plate. The enlargement of the collars is rapid and the delimitation of the pore sites is more or less complete before any significant thickening has occurred in the sieve plate elsewhere. Later, however, the thickening of the rest of the wall overtakes the height of the collars so that eventually the pore sites appear as depressions. The early wall sandwiched between the callose collars remains distinct throughout pore development. The process of perforation seems to involve a more or less simultaneous disappearance of the callose collars and the sandwiched layer. Cisternae of endoplasmic reticulum associated with the pore site seem to be connected with the plasmodesmatal core at all stages of differentiation which adds support to the view that the endoplasmic reticulum plays an active role in the process of pore formation.