Development of the secondary phloem of the primary root of Pisum

Abstract

The divisional pattern of the cambium in giving rise to the secondary phloem and the associated changes in the fine structure of the cellular components from the initial to the morphologically mature sieve element have been described. The fine structure of the sieve element plastid is of particular interest in that it lacks a well-developed internal membrane system but contains two characteristic inclusion bodies, starch granules (which often break up into smaller units) and protein crystalloids (which invariably show subunits with either one- or two-directional periodicity).

Electron microscope autoradiographs were made of cambial cells and their phloem derivatives in tissues exposed either to [³H]thymidine for 24 hr (followed by growth in the absence of tracer for 4 days) or to [³H]uridine for 2-12 hr without a further growth period. The [³H]thymidine labels were specifically lodged in the chromatin material of the cambial initials, the companion cells, the phloem parenchyma cells, and the young sieve elements. During sieve element maturation the [³H]thymidine label became less specifically associated with the chromatin material.

The pattern of labelling of the [³H]uridine in addition to being associated with the nucleolus was also associated with the chromatin, the electron-lucent areas of the nucleus, and the cytoplasm of the young sieve element, the companion cell and the phloem parenchyma indicating that these cells were all active in RNA synthesis. In the sieve element the nucleolus disappeared at a very early stage of development. This was associated with a decrease in [³H]uridine incorporation into the nucleus. As the tonoplast of the sieve element disintegrated, [³H]uridine incorporation into both the nucleus and the cytoplasm stopped, which is interpreted as a cessation in RNA synthesis. At no stage in the development of the sieve element was [³H]uridine incorporated into the "slime" material.