No answer has been found up until now. They work out the contacts between phloem and the surrounding transfusion parenchyma and can also be found as mediators between ray parenchyma cells of the bark and mature sieve cells. Albuminous cells participate in loading and unloading of these cells. The process has been studied in detail in Pinus . Companion cells and sieve elements are of meristematic origin, their development is similar to that of the xylem.
Primary phloem develops by longitudinal division and subsequent elongation of meristematic cells.
The cells do often divide unequally. The bigger daughter cell differentiates into a sieve element, the smaller after one or two further longitudinal divisions into two to four companion cells. Consequently may several companion cells belong to one sieve cell. The exact number is usually typical for the respective tissue, but it may even vary within a single plant. The deposits contain often tannic acids and resins.