A QUANTITATIVE ANALYSIS OF PLANT GROWTH: PART I.

SUMMARY. The series of articles of which this is the first instalment, constitutes an attempt to formulate methods for the quantitative analysis of plant growth and to apply these methods to data which have been lying dormant in the literature for 40 years. In the present chapter the relative growth‐rate curve, which is the weekly percentage increase in dry‐weight plotted against time, and also the leaf‐area ratio curve, that is, the leaf‐area in sq. cms. per g. plotted against time, have been employed. And as a typical example of an annual plant maize has been selected since data are given by Kreusler for this plant grown in four successive years. The first noteworthy result of this analysis is the demonstration of the fact that the growth‐rate varies greatly in magnitude at different periods in the life‐cycle of a plant such as maize in a perfectly definite manner. Fig. 9 gives the generalised form of the growth‐rate curve for maize throughout its life‐cycle. Although the broad form is that of a Sach's grand period curve, it must be noted that it is not a grand period curve, since the grand period curve as denned by Sachs is the curve of the actual increment per unit of time plotted against time and not of relative increment, that is, increment per unit of matter per unit of time plotted against time. On the broad form of the relative growth‐rate curve for maize are superposed three secondary features, an initial fall, and two subsidiary maxima on the descending limb. In this generalised curve the initial period A‐B is the period before the assimilatory organs are able to counterbalance the loss in dry‐weight due to respiration, and the rate of growth is consequently negative or nil. The phase B‐C corresponds to a phase in morphological development during which the leaf‐area per unit dry‐weight increases to a maximum. The phase C‐F covers the remainder of the life‐cycle of the plant during which the leaf‐area per unit dry‐weight is continuously decreasing. The subsidiary maxima D and E coincide with the time of the record of the appearance of the male and female flowers respectively. The minima X, Y which precede these maxima, correspond with the earliest stages of flower development, and are possibly due to increased respiration during that period. The incidence of the maxima is controlled by environmental conditions—not by the environmental conditions operating at the time, but by those obtaining at some previous stage in the life‐history of the plant. The fact that the curve for leaf‐area per unit dry‐weight throughout the season (which has been calculated) shows a correspondence with the growth‐rate curve indicates that the physiological basis for increased and decreased relative rate of growth is a corresponding change in the assimilating area per unit dry‐weight. This point will be dealt with in the next chapter. Evidence from the quantitative analysis of plant growth for maize indicates that the seedling leaves do not perform their normal assimilatory function till some time after their appearance.