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.
Plasmodesmata between sieve element and companion cell of extrafascicular stem phloem of Cucurbita maxima permit passage of 3 kDa fluorescent probes
