Abstract

1. The development of sieve-plate pores was studied by the use of light and electron microscopes. The results reported here pertain to the secondary phloem of Robinia pseudoacacia L. and primary phloem of Cucurbita maxima Duchesne. 2. The sites of the future pores are first delimited by the appearance of small deposits of callose in the form of platelets. The platelets are paired. The two members of a pair occur on two opposite sides of a sieve plate and are separated from one another by the middle lamella (probably compound middle lamella). The platelets increase in diameter, whereas the cellulosic bars between the pore sites-these bars together represent the basic network of the sieve plate-decrease in width. The platelets and the bars increase in thickness. In some sections, plasmodesmata, one to a pore site, may be observed in the young sieve plates before and after the callose platelets develop. Endoplasmic reticulum becomes applied to the pore sites and is associated with the callose until the pores are formed. 3. The perforation occurs in the center of a platelet in such a way that the middle lamella disappears, the callose platelets fuse and break through the fused part. Thus a pore is lined with callose from its inception. Later, callose appears also on the surface of the sieve plate. The perforation of the sieve plates occurs after the nucleus disintegrates and, in Cucurbita, after the slime bodies disaggregate.