The mode of adaptation for drought environmental conditions in rice plant, very complicated by nature as it may be, seems to admit classification into many theoretical types in regard to its physiological characteristics. Among these characteristics, the response of the plant in changing its sap concentration according to the variation of soil moisture or that of concentration of culture solution; mutual transmitting effect of it among parts of plant body; the optimum degree of concentration of culture solutions for the plant growth and its seasonal variation; morphological changes in plant caused by various concentrations, or the general hardening process brought on by restricting water-supply of the soil, etc. may be considered especially of great significance. According to the results of the author's investigations reported in the present paper, combined with those of his preceding works, he is of opinion that these various features of drought adaptation of rice plant may be compositely manifested to a degree by somewhat a simple morphological criterion, that is, the ratio of the height of the plant grown in non-saturated conditions of soil to the height in water-saturated conditions of it-the relative height of plant in "upland" conditions against that in "lowland" ones-say, "R. H. u. to l." in abreviation; and the changes in the latter when experimentally cultured will duely be used as a very convenient diagnostic of drought adaptability of rice plant, just as has already been shown by the same author in his preceeding reports. The main results of the present report are summarized as follows. 1) According to the results of water culture experiments with several rice varieties, the optimum concentration of culture solution differed with the variety, and within each variety, it changed with stages in growth process. 2) Adapting response in the osmotic pressure of sap is very different with the variety. In the case of Kaneko variety, a variety of upland rice, for example, the pressure both in leaves and roots changed with certain rythms in harmony with the concentration of the culture solution; while the other varieties used showed the pressure in each own way and were found almost independent on that of culture solutions. The author is to give the name "Poikilohydre-Pflanzen"-type to the former mode of adaptation, and the name "Homoiohydre Pflanzen"-type to the latter one. 3) Relation of sap concentration changes in the top of plant to those in the root was found various in its degree with the variety, and may be very significant for identification of the type of drought adaptation of the plant. 4) Intensity of morphological contraction of plant caused by high concentration of culture solution was graded into about four classes. In the experiments, the variety Shinagawa, a typical lowland variety, contracted most remarkably; some other varieties of lowland rice showed contraction of intermediate degrees; and among upland rices these were found no variety which contracted so markedly as the lowland ones. 5) The osmotic pressure of plant sap increased as a rule when the plant is cultivated in upland conditions as compared with when in lowland condition; the difference between them of these two cases varies, however, with the type of the variety. Certain upland varieties showed very large differences of concentration as much as about 50 per cent, and may be called "Euryhydre Pflanzen (WALTER)-type, while some lowland varieties showed rather narrower difference as less than 10 per cent only and may be called "Stenohydre Pflanzen (WALTER)"-type. 6) In the experiments on artificial drought by restricting water supply of soil, the hardening processes were observed to consist in two different kinds of adaptation. The first one was chiefly the adaptation by osmotic pressure change, and the second by protoplasmic construction changes. [the rest omitted]

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