[143] Leitch, I., Some Experiments on the Influence of Temperature on the Rate of Growth in Pisum sativum, Ann. of Botany, XXX, pp. 25–46, 1916. (Cf. especially Table III, p. 45.)
[144] Blackman, F. F., Presidential Address in Botany, Brit. Ass. Dublin, 1908.
[145] Rec. de l’Inst. Bot. de Bruxelles, VI, 1906.
[146] Hertwig, O., Einfluss der Temperatur auf die Entwicklung von Rana fusca und R. esculenta, Arch. f. mikrosk. Anat. LI, p. 319, 1898. Cf. also Bialaszewicz, K., Beiträge z. Kenntniss d. Wachsthumsvorgänge bei Amphibienembryonen, Bull. Acad. Sci. de Cracovie, p. 783, 1908; Abstr. in Arch. f. Entwicklungsmech. XXVIII, p. 160, 1909.
[147] Der Grad der Beschleunigung tierischer Entwickelung durch erhöhte Temperatur, A. f. Entw. Mech. XX. p. 130, 1905. More recently, Bialaszewicz has determined the coefficient for the rate of segmentation in Rana as being 2·4 per 10° C.
[148] Das Wachstum des Menschen, p. 329, 1902.
[149] The diurnal periodicity is beautifully shewn in the case of the Hop by Joh. Schmidt (C. R. du Laboratoire de Carlsberg, X, pp. 235–248, Copenhague, 1913).
[150] Trans. Botan. Soc. Edinburgh, XVIII, 1891, p. 456.
[151] I had not received, when this was written, Mr Douglass’s paper, On a method of estimating Rainfall by the Growth of Trees, Bull. Amer. Geograph. Soc. XLVI, pp. 321–335, 1914. Mr Douglass does not fail to notice the long period here described; but he lays more stress on the occurrence of shorter cycles (of 11, 21 and 33 years), well known to meteorologists. Mr Douglass is inclined (and I think rightly) to correlate the variations in growth directly with fluctuations in rainfall, that is to say with alternate periods of moisture and aridity; but he points out that the temperature curves (and also the sunspot curves) are markedly similar.
[152] It may well be that the effect is not due to light after all; but to increased absorption of heat by the soil, as a result of the long hours of exposure to the sun.