On the relations between growth and the environmental conditions of temperature and bright sunshine

SUMMARY. 1 Growth maybe divided conveniently into two well-marked periods. (a) 1st period, from the seedling stage till the time that the plant regains its initial weight after the loss by respiration, i.e. the time during which a casual observer would say the plant ?makes no growth.? (b) 2nd period, succeeding the former, during which the plant is obviously making growth, and which continues till the latter ceases and desiccation sets in. 2 The length of the first period varies inversely with the mean maximum temperature, as the rate at which assimilation is able to make good the loss by respiration increases directly with rise of temperature, up to a certain limit. 3 The possible amount of growth as measured by the dry matter produced depends directly upon the bright sunshine and temperature when the food supply is adequate, but when the latter is limited the total growth is much less owing to the lack of material for building up the tissues. Beyond a certain limit, however, the beneficial factors of heat and bright sunshine become harmful and result in the premature death of the plant. 4 During the first period the rate of growth as shown by the efficiency index was associated with relatively warm days and nights, bright sunshine having little significant effect; the light, however, was good throughout for the season of the year. During the second period the rate was associated strongly with sunshine and warm days, but not significantly with the night temperatures, which did not fall below 32° F. 5 During the greater part of the year the maximum rate of growth (highest efficiency index) is reached early in life, very soon after the second period begins. Under favourable environmental conditions a high rate of increase is then maintained for several weeks, but in less favourable circumstances the efficiency index rapidly falls. In winter, when temperatures rule low and there is little bright sunshine, the maximum rate of growth is not reached till several weeks after the beginning of the second period, and even then the efficiency index is not very great. 6 Plants with a restricted food supply make less total growth than those with abundant food. The falling off in the amount of dry matter produced does not seem to be gradual but is marked by definite periods of which the incidence varies at different seasons. 7 Broadly speaking the response of plants to the environmental conditions is similar whether the food supply is abundant or restricted, though the mean rate of growth is lower when food is scarce. During the first period the excess of food has no significant effect upon the rate of growth, but during the second period the mean differences in the rate of increase in the presence of abundance and of scarcity of food are strongly significant in favour of the well supplied plants. 8 During the early weeks, corresponding approximately to the first period of growth, the shoot/root ratio falls, owing to the steady increase in root weight which is associated at first with a decrease and later with an increase in shoot weight. During the second period of active growth the shoot increases in weight far more rapidly than the root, and thus the shoot/root ratio rises steadily. Increase in shoot growth is closely associated with rise in temperature, though the lowest mean maximum attained in the experiments did not cause a cessation of growth. Root growth is much affected by low mean maximum temperatures and practically ceased, under the experimental conditions, when they were consistently below 60° F. Rise in maximum temperature had much less beneficial action upon the roots than upon the shoots. 9 In early stages of growth the amount of nitrate absorbed by the plant is relatively large in comparison with the dry matter produced, but later on more dry matter is formed in proportion to the same amount of nitrate, owing to the accumulation of the products of assimilation. In conclusion I wish to express my indebtedness to Mr R. A. Fisher, who has examined the figures and has furnished me with the statistical informatio embodied in this paper. In each case the efficiency indices are percent per day.