BOTANY. 781 



temperatures, yet in the i^reseiice of light they can assimilate the car- 

 bonic acid gas of the atmosphere at a temperature of —40^ C, and that 

 the so-called sleep, or vegetative repose, is due to the drying up of the 

 plant and not to the cold. 



Bliznine^ shows that the freezing of wheat in a cold but snowless 

 December does much more injury than in February, apparently owing 

 to the early tender stage of growth. 



Hartig^ shows that the temperature under the bark of a tree after 

 the leaves have fallen, and therefore in winter time, may be 50 or 

 55° C. on the sunny side, while the temperature on the shady side is 

 15 or 20° lees. With this high temperature there comes a stimulated 

 evaporation on the sunny side that separates the bark and kills the 

 tree. 



Petit ^ calls attention to the fact that as water in capillary vessels 

 does not freeze until it is cooled down to a temperature far below that 

 of melting ice, so in like manner the water in the soil which is spread 

 in capillary films over each grain of gravel may be cooled to about 15° F. 

 without freezing, but when it does change to ice it suddenly warms up 

 to 32°. For this reason the temperature of the soil cools more rapidly 

 during the nighttime in proportion as the soil is drier. On the con- 

 trary, in the summer time, when freezing does not come into play, the 

 soil is warmer in proportion as it is drier. 



Prunet* finds that on thawing the frozen plant the water is lost by 

 evaporation from the surface rather than by the ordinary operation of 

 transpiration through the stomata or breathing pores. 



Schindler^ in his extensive si)ecial studies on the relation of wheat 

 to climate accepts the fact that wheat is less sensitive to frost in pro- 

 portion as its vegetative period is longer, but thinks that no explanation 

 is as yet known. The dry wheat of the Steppes does better than the 

 moist wheat of western Europe. The power of resisting frost depends 

 ultimately on the specific peculiarities of the protoplasm, about which 

 we know nothing. 



The effect of bad seasons on the growth of trees, A. Gary ( Gar- 

 den and Forest, 8 {1895), pp. 88, 89). — During the winter of 1893-'9J: the 

 author made a study of the so-called year rings of spruce trees, in the 

 course of which the rings of over 1,400 trees in Maine were counted. 

 In all trees of apparently more than 80 years' growth a belt was noticed 

 in which the rings were greatly reduced in size, in some cases being 

 seen only with the aid of a microscope. By reference to meteorological 

 data of undoubted authority it was learned that a series of remarkably 

 cold seasons began in 1812 and culminated in 1816, the years 1812, 1815, 

 and 1816 being the most severe. The record of the effect of these years 



' Forsch. Gel), agr. Pliys., 15 (1892), p. 122. ^jbid., 16 (1893), p. 64. 



sibid., p. 285. "Ibid., 17 (1894), p. 142. 



^Ibid., p. 209. 



