General Description of Transport Systems in Plants

For a full and detailed description of the vascular systems of plants, the reader is directed to a standard text such as Esau. The movement of solutes within plants takes place largely by two pathways. One route is via the extraprotoplasmic continuum (apoplast) of the plant and includes transport over short distances through the intercellular spaces and over long distances in the xylem vessels. Such transport is normally in an upward direction (acropetal). The second route is via the cytoplasmic continuum (symplast) of the plant and includes short-distance cell-to-cell transport through plasmodesmata and long-distance transport in the phloem sieve cells. Phloem translocation takes place in both upward and downward (basipetal) directions to the sites of new growth. 

The phloem tissue of plants is composed mainly of the sieve elements, companion cells, and the phloem parenchyma. The sieve elements are specialized elongated cells lacking a vacuole and connected at each end to other sieve elements by special pores. However, as in all cells, the cytoplasm is contained within the plasmalemma, with connections to other cells via the plasmodesmata. The phloem carries the products of photosynthesis, the assimilates, from the mature leaves to the meristematic and actively growing parts of the plant. The mechanism of phloem transport has long been the 

As a result of the close proximity of xylem and phloem tissues, there is a potential for considerable lateral transfer of solutes from one transport system to the other. The volume of flow in the xylem, however, is so much greater (perhaps 50- to 100-fold) that solutes that move freely from one to the other will move mainly in the xylem to the mature leaves. 

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