Sites of Accumulation in Excised Phloem and Vascular Tissues

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Abstract
Excised vascular bundle from celery petiole, and excised phloem tissue from apple stem, both readily accumulated phosphate and sulfate from external solution. The paper reports attempts to identify the cellular sites of this accumulation. Tissues were allowed to accumulate radioactive phosphate and sulfate, and were then freeze-dried, sectioned, and autoradiographed. Special methods were needed to prevent any diffusion of the water-soluble radioisotopes. Some autoradiographs obtained are presented. These are interpreted as showing that all the living cells of the phloem accumulated solutes at least 5 times as actively as did the cortical parenchyma cells. The young secondary phloem was the most active tissue, and the sieve tubes were among the most actively accumulating cells. It is concluded that sieve tubes contain a region completely bounded by a semipermeable membrane; and that accumulation processes are directly involved in the mechanism of phloem transport. Results are discussed in relation to some recent studies on the structure of phloem and the function of sieve tubes.