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Translocation of organic substances in the phloem of higher plants has been investigated for almost a century, but remarkably little is known of the mechanisms whereby assimilates are drained from the mesophyll of photosynthesizing leaves and transferred to sieve tubes in the leaf veins. Certain modified companion cells (3, 5-7) associated with the sieve tubes of minor veins have long been suspected of having a special role in this respect (11). Indeed, the term "Uebergangszellen," introduced by Fischer in 1884 (9) to denote these densely staining cells, implies such a function. We shall employ "transfer cells" as a literal translation to preserve the physiological connotation. The present communication is concerned with a specialized type of transfer cell found in certain higher plants, and we wish to present and interpret cytological information relevant to its translocatory function. Fig. 1 shows a cross-section of a vein from a mature leaf of Pisum arvense. Two tracheary elements (x), three narrow sieve elements (s), and two types of phloem parenchyma cell are visible. One type (p) has sparse contents; the other (t) has dense contents. The latter is the transfer cell. Its nucleus is large and usually central (Fig. ). In its cytoplasm are vacuoles, chloroplasts with grana and starch grains, and numerous mitochondria with well-developed cristae (Figs. 7, 8). DictyoG. BRIARTY. From the Department of Botany,

SPECIALIZED "TRANSFER CELLS" IN MINOR VEINS OF LEAVES AND THEIR POSSIBLE SIGNIFICANCE IN PHLOEM TRANSLOCATION