430 PLANT PHYSIOLOGY 



varieties will head very soon, stimulated by the high temperature 

 and long day, while the winter crops, which under such con- 

 ditions cannot pass the stage of yarovisation, mil remain for an 

 indefinitely long period in the tillering phase (Maximov). 



Earliness is thus the result of quahtatively distinct elements, 

 and in breeding plants in this direction it is indispensable to 

 account for all of these elements separately. In crossing varieties 

 according to Lysenko's suggestion, one must not simply take 

 two varieties that have proved early ripening under the given 

 conditions, but the pairs must be so chosen that in one of the 

 parents earliness should be determined, for instance, by a rapid 

 stage of yarovisation, in the other, by a short light stage. In 

 such a way, extremely early varieties may be obtained from two 

 parents, each of which may be a late variety under normal 

 conditions. Systematic selection and breeding of varieties with 

 a definite length of vegetative period are not possible without a 

 previous analysis of the conditions required for these two most 

 important stages of development. Besides, it must always be 

 remembered that varieties early under some conditions may be 

 late under others. 



General References 



Arthur, J. M. et al. The effect of artificial climates on the growth and 

 chemical composition of plants. Am. Jour. Botany, 17: 416-482, 1930. 



Blackman, V. H. Plants in relation to Ught and temperature. Jour. Roy. 

 Hort. Soc, 59: 292-299, 1934. 



BuRKHOLDER, P. R. The role of Ught in the life of plants. Botan. Rev., 

 2 : 1-52, 97-168, 1936. 



Garner, W. W., and H. A. Allard. Effects of the relative length of day 

 and night and other factors of the environment on growth and repro- 

 duction in plants. Jour. Agr. Res., 18: 553-606, 1920. 



Gilbert, B. E. Interrelation of relative day length and temperature. 

 Botan. Gaz., 81: 1-24, 1926. 



HiBBARD, A. D. Photoperiodism and enzyme activity in the soybean plant. 

 Mo. Agr. Exp. Sta. Bull. 271, 1937. 



Hooker, H. D. A survey of investigations by American horticulturists on 

 carbohydrate-nitrogen relations. Jour. Pomol. Hort. Sci., 6 : 34-41, 1925. 



Kraus, E. J., and H. R. Kraybill. Vegetation and reproduction with 

 special reference to the tomato. Ore. Agr. Exp. Sta. Bull. 268, 1937. 



McKiNNEY, H. H., and W. J. Sando. Earliness of sexual reproduction in 

 wheat as influenced by temperature and light relation to growth phases. 

 Jour. Agr. Res., 51: 621-641, 1935. 



Maximov, N. A. The theoretical significance of vernalization. Imp. Bur. 

 Plant Genetics Herbage Ser. Bull. 16, 1934. 



