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A method was developed to determine the extent of penetration of substances into the plant symplast. The known apoplastic dye trisodium 3-hydroxy-5,8,1O-pyrenetrisulphonate (PTS) and various systemic pesticides were studied using potato tuber tissue. The dye penetrated only 5 % of the volume of living tissue while those pesticides which display an apoplastic pattern of transport in plants penetrated the entire tissue volume. The pesticides diffused freely out of the tissue when it was transferred to fresh medium. It is proposed that the term euupoplastic be used to describe chemicals that behave like the dye, and that the term pseudoupoplastic be used to describe chemicals that behave like atrazine, carbendazim, carboxin and diuron. 2,4-D, which displays a symplastic transport pattern in the plant, was concentrated by the tissue and did not diffuse out freely. It appears that the property which allows a pesticide to be transported in the symplast is not its ability to penetrate the plasmalemma but rather its ability to be retained by the symplast after entry.

A theory is presented to explain the phloem mobility of certain systemic xenobiotics that are not weak acids. It is shown that there is a theoretically optimum permeability that permits optimum circulation through the symplasm and apoplast (including the phloem and xylem) of Solanum tuberosum plants. The optimum permeability is large enough to permit substantial passive permeation into sieve cells in the source leaf and yet is small enough to permit phloem ransport with some retention. The optimum permeability is a function of the velocity of sap flow in sieve tubes, the radius of the sieve tube, the over-all length of the plant, and the length of the carbohydrate and xenobiotic sources. It is argued that the nematicide, oxamyL is near the optimum permeability under some experimental conditions. It is shown that depending on the strength of the carbohydrate sink in roots or growth points and depending on the permeability of the xenobiotic, there can be passive accumulation of xenobiotics in the sieve tubes in the carbohydrate sink regions.The patterns of translocation which various xenobiotic substances display after entering a plant have been described for many chemicals, especially herbicides (1). Two distinctive patterns of transport were discerned, namely the apoplastic pattern and the symplasmic pattern. Movement in the apoplast occurs mainly via the xylem and the cell walls of the parenchyma cells, while movement in the symplasm occurs via the phloem and the interconnected protoplasts of the parenchyma cells. Chemicals moving by both routes are called ambimobile (5). It has been assumed that the potential for a chemical to penetrate the plasmalemma of the cells is correlated with the over-all pattern of translocation it achieves in the plant as a whole. It was originally thought that chemicals displaying an apoplastic transport pattern were unable to penetrate the plasmalemma of plant cells and thus were excluded from the plant symplasm but recent investigations have shown that several chemicals termed apoplastic do penetrate plant cell membranes (4, 7-10, 13). Peterson and Edgington (7) have called such chemicals pseudoapoplastic. They suggested that the permeability of the plasmalemma to the pseudoapoplastic chemical is so great that it cannot be retained in the symplasm for long

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