590 FACTORS AFFECTING GROWTH 



of temperate zone origin do not grow appreciably at temperatures below 5° C. 

 Their optimum growth temperature is usually about 25-30° C, and their 

 maximum about 35-40° C. The cardinal growth temperatures of most tropi- 

 cal and sub-tropical species are still higher. For maize, a crop plant of sub- 

 tropical origin, the minimum growth temperature is about 10° C, the opti- 

 mum about 30-35° C, and the maximum about 45° C. 



Leitch ( 1916) has made one of the few comprehensive studies of the effect 

 of temperature upon the quantitative aspects of growth (see also Lehenbauer, 

 19 14). She found that the rate of elongation of the roots of pea seedlings 

 increased consistently with rise in temperature in the range of —2° to 29° C, 

 and further that the rates within this range of temperatures, once established, 

 showed little or no diminution with time. Above about 30° C, the higher 

 the temperature, the lower the initial rate of growth, and the more rapidly the 

 rate decreased with time. Elongation ceased entirely at temperatures of 

 45° C. or higher. In other words in the temperature range of 30-45° C, a 

 distinct time factor effect was evident in the relation between temperature and 

 growth. 



The optimum temperature for the enlargement or elongation phase of 

 growth is seldom the optimum for other phases of growth. Each stage in 

 the development of a plant may and usually does have a different optimum 

 temperature. In many species the optimum temperature for the germination 

 of seeds is less than for vegetative growth, which in turn is often lower than 

 the most suitable temperature for flowering and fruiting. 



2. Temperature Limitations upon Plant Survival. — A clear distinction 

 should be drawn between the extremes of temperature at which the growth of 

 a plant ceases, and the extremes of temperature which that plant can endure 

 without death resulting. The minimum temperature which a plant can 

 tolerate without injury is almost invariably below that at which growth 

 ceases; likewise plants can usually endure without lethal effects, at least 

 temporarily, temperatures considerably in excess of the maximum at which 

 growth occurs. A given plant, for example, may cease to grow when the 

 temperature to which it is exposed rises to 40° C. Death, however, occurs 

 only if the temperature of the plant is raised to some still higher temperatures, 

 perhaps 55° or 60° C. Within this intervening range of temperatures the 

 plant passes into a state of heat rigor in which it neither grows nor exhibits 

 growth movements. Similarly there is a range of temperatures between the 

 lowest temperature at which a plant will grow and its death point due to 

 cold. Within this zone of temperatures the plant passes into a corresponding 

 condition of cold rigor. 



The upper and lower extremes of temperature which plants or plant organs 



