Symplasm/symplast

An ion entering a root may immediately enter the symplast by crossing the plasma membrane of an epidermal cell, or it may remain in the apoplasm and diffnse throngh cell walls. It may snbseqnently enter the symplasm by crossing... 

Edgington and Peterson (4) have subdivided apoplastic xeno-biotics into two classes. Euapoplastic (only transported in the apoplast) and pseudoapoplastic (transport occurs mainly in the xylem but entry into the symplast occurs). Most traditional "apoplastic" chemicals are now known to really be pseudoapoplastic chemicals, e.g., atrazine, diuron, oxamyl, etc. The unresolved question is why don t these pseudoapoplastic chemicals which cross the cell membranes and enter the symplast remain in the symplasm of the phloem There have been numerous studies focusing on the molecular requirements for phloem mobility (1-5), In general, there is not a good correlation between phloem mobility and water solubility, metabolism of the xenobiotic, or the presence of various substitution groups in a molecule. 

In fact, membranes generally serve as the main barrier to water flow into or out of plant cells. The interstices of the cell walls provide a much easier pathway for such flow, and hollow xylem vessels present the least impediment to flow (such as up a stem). Consequently, the xylem provides a plant with tubes, or conduits, that are remarkably well suited for moving water over long distances. The region of a plant made up of cell walls and the hollow xylem vessels is often called the apoplast, as noted above (Chapter 1, Section 1.1D and in Section 9.4A). Water and the solutes that it contains can move fairly readily in the apoplast, but they must cross a membrane to enter the symplast (symplasm), the interconnected cytoplasm of the cells. 

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