GROWTH 



299 



between thermophilous plants with very high cardinal points and 

 plants resistant to low temperatures, but more sensitive to exces- 

 sively 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 

 Degrees Centigrade 



Plant 



Barley, oats, rye, wheat 



Buckwheat 



Hemp 



Sunflower 



Maize 



Pumpkin 



Melon, cucumber 



Maximum 



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 car- 

 dinal points, but the latitudes of temperature between each, are 

 different. In general, the discrepancy between the minimum and 

 the optimum is always considerably greater than 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 tem- 

 perature begins to have an influence. 



The minimum and maximum points are the temperatures at 

 which growth is checked, 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, 

 ultra maximum, is somewhat above the maximum for growth. 

 Between these points there is a stage at which growth ceases, a 

 peculiar inhibition of the organism. Plants can withstand tem- 

 peratures in this range without injury, if exposure does not continue 

 too long. A prolonged retardation especially from excessive heat 



