DEVELOPMENT OF CHAMBERED PITH IN STEMS OF PHYTOLACCA AMERICANA L. (PHYTOLACCACEAE)

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Abstract
An integrated microscopic (light and electron microscopy) and macroscopic investigation of chambered pith development was made of Phytolacca americana L. Terminal internodes have a solid pith cylinder in contrast to the alternating diaphragms and chambers occurring in subjacent pith. Macroscopically, chambers and diaphragms of any one internode are of equal size. Microscopically, diaphragms vary in height within an internode (from 1–6 cells high). Nevertheless, all diaphragms become thicker circumferentially (5–12 cells high) and connect with long files of intact peripheral pith cells. Diaphragm cells have a large centrally positioned vacuole with a thin, parietal layer of cytoplasm; nuclei, mitochondria, endoplasmic reticulum, and unidentified organelles differentiate in the cytoplasm of diaphragm cells. Although schizogenous activity has most often been implicated as the mechanism by which chambered pith develops in vegetative organs of angiosperms, the results of this study show that cavities in pokeweed result from both schizogenous and lysigenous mechanisms. Schizogeny is suggested by the fact that central pith cells of terminal internodes are longer and thinner walled than peripheral pith cells arranged in vertical files, thus indicating elongation of cells as a possible result of internode elongation. The precise developmental pattern and arrangement of chambers and diaphragms also suggest schizogenous processes. Lysigenous or enzymatic activity is indicated by the fact that cavities are bounded by broken cells, and wall fragments and organelles are often found within enlarging cavities. Chamber formation occurs continuously acropetally and centrifugally in the central pith. A comparison of diaphragms is made with Liriodendron tulipifera and Juglans nigra in an attempt to resolve differences in structure and terminology regarding the differentiation of chambered and diaphragmed pith.