Ontogeny of cambium and Phloem in the Epicotyl of Pisum sativum

Abstract

The pattern of distribution and differentiation of the primary phloem, the cambium, and the secondary phloem, and the exact pattern of division of the initial cell and its derivatives have been studied in the epicotyl of pea plants by using electron microscopy.

Three divisional patterns of the initial cell, in giving rise to the phloem cells, are recognized. The initial cell first divides periclinally to give rise to a transitional cell. This transitional cell then divides further (periclinally and/or anticlinally) to give rise to three sequences of phloem derivatives: (1) phloem parenchyma cells, (2) a companion cell and a sieve cell, and (3) a companion cell, a sieve cell, and a phloem parenchyma cell. The derived cells can all easily be distinguished from each other either by their position in the vascular bundle at low magnification or by the different types of plastids present in them.

The general pattern of differentiation of the cytoplasm and the formation of the sieve plate and the sieve pores of the sieve element are essentially similar in the primary and the secondary phloem. However, the sieve element of the secondary phloem, unlike that of the primary phloem, possesses in its cytoplasm three kinds of inclusion bodies - an amorphous form, a "crystalline" form, and a tubular form; these are described and their nature discussed.