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This investigation is essentially a collaboration between two laboratories bringing together problems in which each is interested. The Department of Agricultural Research of the American Smelting and Refining Company has long been interested in the problems of sulphur toxicity and sulphur metabolism in plants which are economically important. In the course of this work investigations have been made by the use of SS5. These have shown the general distribution of the isotope in the plant body. To some extent chemical procedures have been employed to identify in the protein hydrolysate specific sulphur-containing compounds, notably cystine and probably methionine (8, 9, and 10). It remained, however, necessary to obtain unequivocable evidence on the sulphur compounds into which the isotope had entered. In the Department of Botany at Rochester, investigations have been in progress since 1946 utilizing the techniques of paper chromatography to identify the free and combined amino acids in plants (2, 6, and 7). The sulphur-containing amino acids present rather special problems. Due to the small quantities commonly present and the poor reactivity of these substances with ninhydrin, they tend to be less easily detectable by the methods of paper chromatography than many of the other amino acids (1). Furthermore, as two-directional paper chromatograms are commonly carried out (i.e., using phenol : collidine-lutidine), the sulphur-containing amino acids tend to be obscured by others which are commonly present. Methionine sulphoxide occupies the same area as ?-amino butyric acid, a newly discovered free amino acid which is widely distributed (4, ?, and 7). Methionine itself is close to the leucines, and cystine can best be recognized if it is first converted to cysteic acid which occupies a position on the chromatograms well apart from other amino acids (1). In these circumstances the use of S85 combined with autoradiographs of paper chromatograms is an obvious device to aid in the identification of the sulphur-containing amino acids. In the laboratories of the American Smelting and Refining Company alfalfa plants were treated with S35 in the form of sulphate. The alfalfa

This is the first of a group of papers which will record investigations on nitrogen compounds and their metabolism in plants. The work was undertaken not only because of the intrinsic interest of nitrogen metabolism, but even more because it had become so apparent that other vital phenomena would only be fully comprehended when their relations to nitrogen metabolism were revealed (26, and refs. there cited; 27). Plant respiration?long considered in terms only of carbohydrate breakdown?has recently been shown to include a moiety which varies concomitantly with protein synthesis and the metabolism of amino acids. This was especially true of the cells of the potato tuber (24), under the conditions which promote accumulation of salts. In another series of papers (for references see 21), the ability to accumulate salts, now somewhat generally conceded to be a property of active cells and dependent upon their aerobic respiration, has also been associated in a variety of ways with cells able to grow and to synthesize protein and with factors able to foster this process (22, 23, 24). For the particular case of the potato tuber, it was recognized that aerobic respiration, protein synthesis from the stored reserves of soluble nitrogen and the accumulation of anions (Br) and cations (K) varied pari passu under the influence of a wide range of conditions. Observations such as these led to the view that the energy of respiration was made available simultaneously for both salt accumulation and the protein synthesis with which it appeared to be linked. The necessary point of contact between the energy derived from aerobic respiration with the metabolism of nitrogen compounds and salt accumulation, promised, therefore, to be fundamental to the understanding of all of these processes. Prior to the work now to be described, some progress had been made by analytical methods which could fractionate the soluble nitrogen of the cells only into the amino-N and amide-N fractions (22). The total amide-N was further separable into glutamine-amide-N and asparagine-amide-N (25), and it became apparent that these two substances stood in quite different relationships to protein synthesis (26, and refs. there cited). Further

Suspensions of chloroplasts isolated from leaves of spinach or peas and illuminated in suitable aqueous media are capable of both 02 evolution and CO2 fixation at rates comparable with those which occur in vivo (8, 11). Under these conditions the major radioactive products of photosynthetic "4CO2 assimilation are glycerate-3-P and dihydroxyacetone-P, a considerable proportion of which may be recovered from the medium outside the chloroplasts (3). It is still not clear what prevents metabolism of these early products of photosynthesis to hexose phosphates and compounds such as amino acids and sucrose. Recently we have suggested (6) that glycerate-3-P and DHAP1 accumulate in chloroplast suspensions simply because they diffuse from the plastids into the external medium where they cannot be metabolized further. We have shown that these compounds were not accumulated if the volume of medium in the experimental sample was reduced to a minimum by layering the chloroplasts on filter paper discs. Under these conditions the chloroplasts accumulated hexose-monoP and amino acids such as glycine and alanine but did not form significant amounts of sucrose. Factors controlling the biosynthesis of sucrose from CO2 in chloroplasts remain unknown. It is possible that the nature of the products formed by isolated chloroplasts could depend on the relative concentrations of intermediates of the photosynthetic carbon reduction cycle within the plastids. We have therefore investigated effects of preincubating chloroplasts with varying concentrations of glycerate-3-P and sugar phosphates on the distribution of radioactivity into the products of 14CO2 assimilation by spinach

Wright's model for the effects of random fluctuations in gene frequency in a population of fixed size is generalized to randomly fluctuating population size, and treated from the viewpoint of G. Malécot, using a martingale convergence theorem. The gene frequency approaches a limit, whose value depends on the actual realization, or history, of the process; that is, convergence is with probability one (or: almost surely) in statistical language. The limit does not necessarily represent a state of fixation of either allele; in particular, the limiting probability distribution is not necessarily trivial. For the special case of deterministically varying population size, a necessary and sufficient condition for such non-triviality is given.