GROWTH OF PLANTS 



129 



A distinction may be made between thermophilous plants with 

 very high cardinal points and plants resistant to low tempera- 

 tures but more sensitive to very high ones. An absolutely precise 

 determination of these cardinal points for higher plants presents 

 great difficulties, since the influence of temperature is usually 

 intricately connected with the influence of other external factors. 

 An approximate idea of the temperature requirements of different 

 plants may be gained from the table of germinating temperatures 

 for seeds, compiled some fifty years ago by Haberlandt. 



Cardinal Temperature Points for Growth of Seedlings in Degrees 



Centigrade 



Plant 



Barley, oats, rye, wheat 



Buckwheat 



Hemp 



Sunflower 



Maize 



Pumpkin 



Melon, cucumber 



Maxi- 

 mum 



31 to 37 

 37 to 44 

 44 to 50 

 37 to 44 

 44 to 50 

 44 to 50 

 44 to 50 



The table shows that not only the absolute position of the 

 cardinal points, but also the range of temperature between each, 

 are different for different plants. In general, the discrepancy 

 between the minimum and the optimum is always considerably 

 greater than that between the optimum and the maximum. The 

 former is usually 25 to 30°C., while the latter does not exceed 

 5 to 10°C. This seems to indicate that beyond the optimum 

 the injurious influence of high temperatures quickly becomes 

 evident. 



The minimum and maximum points are the temperatures at 

 which growth is stopped but which do not cause the death of the 

 plant. The minimum for life, the so-called ''ultraminimum," 

 is usually below the minimum for growth; while the maximum 

 for life, ''ultramaximum," is somewhat above the maximum for 

 growth. Between these latter points, there is a stage at which 

 growth ceases, a pecuhar inhibition of the organism. Plants can 

 withstand temperatures in this range without injury, if exposure 

 does not continue too long. A prolonged retardation, especially 



