SUMMARY.In this chapter we have continued our analysis of the results of the experiments on the growth of maize carried out by Kreusler and his co‐workers. The rate of growth has been expressed per unit leaf‐area instead of per unit dry‐weight as in the last chapter. The term “Unit Leaf Rate” is used for the weekly rate of increase of dry‐weight in mgs. per sq. cm. The Unit Leaf Rate, instead of undergoing a perfectly definite type of variation, as does the Relative Growth Rate, fluctuates about a mean value. The larger fluctuations which occur in the values for Unit Leaf Rate calculated for the later phases of the life‐cycle have been attributed mainly to sampling errors.Correlations between Unit Leaf Rate and various environmental factors have been determined.The general evidence is that the Unit Leaf Rate is correlated more closely with temperature than with any of the other environmental factors.By allowing for respiration on the basis of our own experimental results values for the real assimilation were arrived at. These also show a closer correlation with temperature than with light.The values for assimilation determined from the Unit Leaf Rate are of a lower order than those determined by the “half‐leaf” method, but much higher than those determined by the “gasometric” method.Finally, the authors wish to express their indebtedness to Dr F. F. Blackmail for his stimulating criticism and help in this and in the previous chapter.

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.

Summary The trilobed caudex of Isoëtes japonica consists of two distinct structures, viz. Stem and Rhizophore, to which the leaves and roots are respectively attached; but owing to the stunted growth of the plant, all external morphological differentiation between the two organs has been completely lost. Stem.—The stem‐apex has the form of a conical mass of tissue situated at the base of the funnel‐shaped depression in the cortex. In this protuberance no definite apical cell can be distinguished. Primary xylem, phloem, and cortex are differentiated from the primary meristem of the stem. The eauline primary vascular axis is a non‐medullated monostele. Primary phloem, in which true sieve‐tubes occur, surrounds the central xylem‐core. An endophytic mycorrhiza is found in the peripheral cells of the primary cortex. The cambium, which arises very early from the outermost layer of the plerome, cuts off secondary cortex externally and secondary phloem internally. Sieve‐tubes with sieve‐areas of the typical cryptogamic type occur both in the primary and in the secondary phloem, and are continuous with those of the leaf‐traces. No secondary xylem is formed in Isoëtes japonica. Rhizophore.—The roots, the vascular bundles of which are collateral with usually endarch protoxylem, are arranged in acropetal series upon a distinct root‐bearing organ, the rhizophore, which in this genus must be regarded as an organ sui generis. The primary growth of the rhizophore proceeds from a primary meristem situated along three radiating lines which correspond to the main fissures in the caudex. The primary and secondary tissues of the rhizophore are essentially similar to the corresponding tissues of the stem. Leaf.—The protoxylem of the collateral vascular bundle is exarch in the lamina, but becomes mesarch in the sporangial region of the leaf. True sieve‐tubes occur in the phloem of the leaf. The ligule is very well developed in Isoëtes japonica. It has a protective function; the young ligule envelops the younger leaves and also secretes mucilage. Systematic.—The species of Isoëtes can be grouped together under two sections, Eu‐Isoëtes and Cephaloceraton. Isoëtes occupies an isolated position amongst recent Vascular Cryptogams, and is regarded as the sole living representative of the Class Isoëtales. In conclusion, we wish to express our thanks to Professor J. B. Earmer, F.R.S., for his valuable advice and kindly criticism throughout the course of this investigation.

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In Part I of this series (9) it was shown that the respiratory activity of an apple gathered at "maturity" in the autumn and subsequently kept at a constant temperature was characterized by a rise followed by a fall. This rise in respiratory activity was attributed to a change of state in the protoplasm. It was considered to mark the onset of senescence and was termed the "climacteric. " It is interesting to inquire whether the senescent rise occurs in fruit picked in an immature condition.Outline of the main experiment, 1925 In 1925, Bramley's Seedling apples were taken at intervals during their period of growth from selected trees in an orchard near Cambridge. They were immediately brought into the laboratory and placed in glass containers lheld at a constant temperature of 12°C. and ventilated by a constant stream of C02-free air of constant humidity. Under these conditions the rate of CO2 production was followed for as long a period as the fruit remained sound; i.e., free from the attack of fungi. The details of the various gatherings are given in table I. The temperatures in the orchard are given in table II.The glass containers used in these experiments were approximately 700 ml. in volume and were originally designed to hold one fully grown apple. Single apples were the experimental units in the case of the later gatherings. In the case of the earlier gatherings the experimental units consisted of more than one apple (table I). They were ventilated at a rate of about 4 lilers an hour, the incoming CO2-free air being passed through soda lime and through a wash bottle containing a 7 per cent. solution of sodium hydroxide. The number of fruits per container was so arranged that the rate of evolu-1 This is the second of a series of papers on the general subject of "Physiology of PLANT PHYSIOLOGY tion of carbon dioxide per container should be as far as possible the same in all experiments, and in any case not more than 1.5 ml. per hour.The measurement of carbon dioxide was made by the Pettenkofer tube absorption method, using N/10 barium hydroxide as the absorbent. Subsidiary to the main plan of the research, as stated above, a number of additional experiments of an exploratory nature were conducted. The whole apparatus was ilnstalled in a constant-temperature room at 120 C.In operation the carbon dioxide produced by the apples is absorbed by the sodium hydroxide; the oxygen consumed by the apples causes a fall in pressure. Oxygen is then passed in from the graduated burette until the manometer shows that the pressures in the two chambers are again balanced. In starting an experiment the first reading is taken after putting in the apple and allowing an hour to elapse before closing the taps connecting both the apple chamber and balance chamber to air. The barometric reading is taken at this time and the volume reading on the oxygen burette, with the manometer connecting the balance chamber and the apple chamber at zero. The tap connecting the oxygen burette and apple chamber is then closed. Every ten hours...