RECENT ADVANCES IN SCIENCE 395 



with increasing temperature. Even between o° C. and 2 8° C. 

 the van't Hoff rule only holds over a small range of tempera- 

 ture, the temperature coefficient decreasing with rise of tem- 

 perature. A more extended series of measurements has 

 been made by I. Leitch (Ann. of Bot. 30, 25-46, 1916), who 

 measured the rate of growth of the main root of Pisum 

 sativum at different temperatures. This author's experiments 

 were of two kinds, long-period experiments extending over 

 22*5 hours, in which the increase in length of the roots was 

 measured macroscopically, and experiments in which measure- 

 ments were made every half-hour with the aid of a microscope. 

 The author concludes from her experiments that the relation- 

 ship between temperature and growth does not obey the 

 van't Hoff rule, but between— 2° C. and 29 C. agrees with the 

 curves found by Krogh for " standard metabolism " (see 

 Krogh, Respiratory Exchange of Animals and Man, London, 

 191 6) and by Keuper for respiration in Pisum. 



Above 29 C. it is impossible to construct a curve to show 

 the relationship between temperature and growth, because the 

 latter decreases with time. Consequently between 30 C. and 

 40 C. time-growth curves must be constructed for each tem- 

 perature. Above 44-5° C. there is no growth. 



From among observations made in the field those of H. N. 

 Vinall and H. R. Reed (Journ. Agric. Research, 13, 133-47, 

 191 8) may be selected. These observations have been made 

 on the American crop plant Sorghum. This plant is semi-tropical 

 and will not grow well if the temperature is too low. On the 

 other hand there is a marked contrast between its growth 

 in California and in Washington, although the mean tempera- 

 tures and " total positive heat units " available in the two 

 localities are not much different. The differences appear to 

 be ascribable to differences in illumination. On the other 

 hand, temperature has a marked influence on flower and fruit 

 formation, higher temperatures during flowering and fruiting 

 depressing the yield of grain. 



W. B. McDougall (Amer. Journ. Bot. 3, 384-92, 1916), 

 working with forest trees, finds that growth starts in the spring 

 as soon as the temperature of the soil is sufficient for root- 

 absorption to commence, and ceases in the autumn when the 

 soil becomes too cold. Sometimes there is a summer resting 

 period, but not always. When this does happen it is due to 

 decrease in water-supply, and is not a periodicity phenomenon. 



From among other work dealing with humidity of the 

 soil may be mentioned that of H. A. Allard (Amer. Journ. 

 Bot. 3, 493-501, 1 91 6), who found that plants which had 

 become stunted through growing in dry soil nevertheless bore 

 as many leaves as properly developed plants. On the other 



