Secondary phloem is important in long-distance transport of organic solutes from the "source" to the "sink," but the thickness of the tissue is never very great.
Shortly after the cambium develops around each spoke of the patterned primary xylem, it soon rounds out to form a circle, laying down new secondary xylem to the inside and new secondary phloem to the outside.
Plants with successive cambia, such as Beta, the beet, produce rings of vascular tissue, each with secondary phloem, a cambium, and secondary xylem, separated from each other by conjunctive tissue.
a-b Jacaratia hassleriana Chodat (Caricaceae), Arid Lands Greenhouses, a Transverse section; secondary xylem of stem, fibers lacking in xylem; fibers present in secondary phloem at top.
Histochcmical and microscopic features of secondary phloem (Table 2, Fig.
Secondary phloem of 1.08 mm and peridem 0.95 mm wide.
Cambial activity within the strands leading to the formation of new secondary phloem
cells (Carlquist 2013).
More importantly, each of the successive vascular cambia continues to produce secondary phloem
for long periods of time, suggesting that vessels function for longer, and thus a greater actual conductive area is achieved in a given stem without adding very many vessels in newer vascular increments.
The monocot secondary meristem produces most of the derivatives inner to the cambial cylinder with both the secondary phloem and secondary xylem lying on one side, which makes the growth essentially unidirectional (Philipson & Ward, 1965).
Storied structure of the secondary phloem. IAWA Bulletin 7: 47-51.
Characterization of secondary phloem in members of Cactoideae is scarce (Mauseth & Ross, 1988: Loza.
In both analyses, Neobuxhaumia and Cephalocereus are sister taxa, based on the occurrence of prismatic crystal in the epidermis (36) and the lack of sclereids in the secondary phloem (52).