STUDIES IN TRANSLOCATION

Electron.microscope studies have been made of the cells of the phloem and pericyclic ground parenchyma of the stem of Cucurbita pepo (Duchesne). The mature sieve element contains no cytoplasm, either on the side walls or at the sieve plate, such as occurs in other cells. The walls are lined with a parietal layer which encloses an aqueous solution in which are dispersed fibrils of slime. The parietal layer appears to be composed of from one to several membranes, with which are associated numbers of vesicles and a very few mitochondria. The parietal layer lines the sieve plate and extends as a lining to the open sieve pores. There is no closing layer or membrane across the sieve pores. Slime is dispersed more or less uniformly through the lumen, and is continuous through the pores. Dense connecting strands of slime are considered to be an artefact. The individual elements are continuous with each other via the open sieve pores, forming a conduit, the sieve tube. The absence of cytoplasm and organelles suggests that the sieve tube IS metabolically inert. The companion cells, in contrast, are rich in cytoplasm. They are packed with mitochondria and other inclusions, and appear capable of high levels of activity in several aspects of metabolism. The phloem parenchyma cells are similar in appearance to the pericyclic parenchyma cells and are much less rich in cytoplasmic content,s than the companion cells.