Not only are proteins very closely connected with the phenomena of life, but unlike other essential constituents of the living cell, they are in all probability characteristic of the animal or plant species in which they are found. The physical properties of the proteins apparently adapt them to the performance of functions essential to the activities of life. Notwithstanding this unique position which these nitrogenous compounds occupy in the plant metabolism, but little is known as to the part they play in the metabolic cycle of the higher plants.Owing to the difficulties involved in separation and identification of various nitrogenous compounds, earlier investigators could not proceed much beyond the determination of total nitrogen in their material. Recent advances in protein chemistry have made a further study of these nitrogenous compounds possible. OSBORNE'S work on proteins in spinach leaves (7) closely followed by a series of papers by CHIBNALL on proteins in leaves of various annuals, including studies on runner bean leaves (3), stimulated interest in the study of nitrogen metabolism of the higher plants by nitrogen fractionation methods. THOMAS (8) was the first to undertake a rather complete nitrogen fractionation of woody tissue like apple twigs.This work was undertaken to determine the seasonal variations in the nitrogenous compounds in the bark and the wood of the current year's growth in Bartlett pear trees. This paper deals with fractionation of total nitrogen into water-soluble and insoluble nitrogen and further division of water-soluble into soluble protein and non-protein nitrogen. Fractionation of soluble non-protein and insoluble nitrogen will be taken up in later papers.Material The samples were taken from twelve 6-year-old Bartlett pear trees grown at the University Farm, Davis, California. These trees were fairly uniform and may be considered as representative trees for their age on a well-managed farm in that locality.Samples were collected at intervals of approximately one month from

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Three problems have been investigated in connection with removing the nitrogenous compounds from pear tissues: first, a suitable method for preservation of tissues previous to such an extraction; second, methods for obtaining an aqueous extract; third, the nitrogenous fractions found in expressed sap. In general the total N extracted has been divided into proteins and non-proteins, which because of the lack of exact separation are referred to as "proteins" and "non-proteins." Apple and spinach tissues were also included in the investigation for the purpose of comparison. As it was not possible to demonstrate the presence of nitrates in such pear tissues as were used, the simple Gunning method for the determination of total nitrogen of the A. 0. A. C. was used in all cases. For the sake of brevity reference has been made to previous work in this field only when it seemed necessary.I. The influence of preservation on distribution and extractability of nitrogenous materials of pear tissues The treatments investigated were of two types: (1) those which killsuch as desiccation, freezing, and the use of chemical cytolizing agents; (2) treatments permitting the tissues to remain alive, such as moderately low temperatures. Besides these treatments, which were intended to minimize chemical change, others were tried which were considered not to be so favorable. These latter were intended to show the influence of two disturbing factors, autolysis, and heat coagulation of the proteins. The several maltreatments used were the storage of minced tissues, the incubation of such tissues, and the exposure to coagulating temperatures during desiccation. MATERIALS AND METHODSThe leaves used were of the Beurre Hardy variety collected in the early summer. The other tissues were bark and wood from the basal portion of vigorous-growing Bartlett shoots. The investigation covered a period of two years. The second year the same tissues were used but the N content of all was higher because the samples were taken from younger trees. The leaves for 1928 appeared to contain more N because the petioles were not included. Usually bark and wood were separated from each other at the beginning of the various treatments. In one case the twigs were frozen with the bark on the wood, and the separation made after thawing.

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