THE SYNTHESIS of starch, the most important reserve carbohydrate in plants, is a problem which has commanded considerable attention. A major advance in understanding starch transformations was the discovery by Hanes (1940a, b) that the linear fraction of starch may be synthesized in vitro by phosphorylase prepared from peas or potatoes. Later, an enzyme concerned with the synthesis of the amylopectin component of starch was reported by Haworth et al. (1944), Bourne and Peat (1945), and Bernfeld and Meutemedian (1948a, b). The mechanism of action and properties of purified phosphorylases have been discussed in a number of excellent reviews (Cori and Cori, 1946; Hassid, 1946; Haworth, 1946; Kalckar, 1945; Peat, 1946; Wynne, 1946; etc.) Certainly more is known about the in vitro reaction of starch synthesis by cell extracts than its synthesis within the extremely complex environment of the plant cell; the reactions occurring in vivo may not be entirely comparable to reactions with cell extracts. Yin and Sun (1947) developed a histochemical stainino procedure by which the presence of phosphorylase can be detected in plant tissue. Using this technique, they studied its physiological role in the living plant by correlating the distribution of the enzyme with the site of normal starch synthesis in germinating soy beans (Yin and Sun, 1949). Still more should be done to elucidate the role of phosphorylase in plant cells, and it was the purpose of the present work to investigate further some aspects of starch synithesis in vivo and in situ. MATERIALS AND METHODS.-From a variety of plant parts examined for phosphorylase activity, the primary root tips of pea seedlings were selected as the test material. They demonstrated fairly consistent activity, and the presence of stored starch, which occurred only in the root tip of very young seedlings and to a limited extent in the root cap, was not a problem. The pea seeds (Wisconsin Early Harvest, Olds Seed Co.) were soaked and aerated, germinated between damp paper towels until the radicles were 2-3 cm. long, and then transferred to aerated mineral solutions in the dark. Seedlings 10-14 days old were usually used. The method of Yin and Sun (1947) in which free-hand sections are incubated in buffered glucose-l-phosphate solution was modified for many of the experiments bv cutting longitudinal sections 40-60 V. thick on a freezino microtome. When it was desirable to avoid injury to the cells from freezing,