54 SOIL CONDITIONS AND PLANT GRO WTH 



time throughout. It varies even for different varieties of the 

 same crop; plant breeders are continually trying to evolve 

 strains suited to particular ranges, e.g. wheats have been bred 

 at Ottawa to ripen in the northern parts of Canada. 



2. It affects the lengths of the periods of vegetative 

 growth and of maturation and therefore causes certain modi- 

 fications in the plant itself. 



A long ripening period gives wheat a plump kernel with 

 a low percentage of protein, while a short ripening period 

 gives an increased protein content (Lawes and Gilbert in 

 1857, 1 66c). 



Turnips in the south of England not only make less growth 

 than in the north, but have a somewhat different composition. 

 Oat straw in Scotland differs in composition from that in 

 England, the translocation of material to the grain being 

 apparently less complete. 



3. The temperature at the time of ripening profoundly 

 affects the germination capacity of the seed.^ 



Light. 



Although light is not a soil factor it nevertheless indirectly 

 affects the soil by modifying the flora which, as we shall 



1 For a discussion of the physiological effects produced by temperature see — 

 Action dc la Chaleur et dufroid sur VactivitS des etres vivants : par Georges 



Matisse. Paris (Larose), igig. 

 Bull. Internat. Inst. Ag. Rome, 1917, 8, 340. 

 Darwin, Francis, On the growth of the fruit of '^ Cucurbita" {Annals of 



Botany, 1893, 28, 459-487). 

 Leitch, I., Some experiments on the influence of the rate of growth of " Pisum 



sativum " (Ann. Botany, 1916, 30, 25-46). 

 Lehenbauer, P. A., Growth of maize seedlings in relation to temperature 



{Physiol. Researches, i, 247-288, 1914). 

 Lepeschkin, W. W., Zur Erkenntnis der Einwirkung supramaximaler 



Temperaturen auf die Pflanze {Ber. Deufsch. Bot. Gesell., 30, 703-714, 



1912). 

 Maximow N. A., Chemische Schutzmittel der Pflanzen gegen Erfrieren {Ber. 



Deutsch. Bot. Gesell., 30, 52-65, 293-305, 504-916, 1912). 

 Groves, J. F., Temperature and life duration of seeds {Bot. Gazette, 63, 



169-189, 1917). 

 Livingstone, B. E., and Livingston, Grace J., Temperature Coefficients in 



Plant Geography and Climatology {Bot. Gazette, 1913, $6, 349-375. Abs, 



in Journ. Ecol., 1914, 2, 179). 



