256 PHYSIOLOGY OF GROWTH AND CONFIGURATION 



that the minima, optima and maxima of temperature are not the same for differ- 

 ent plants. The differences between the various minima are especially strik- 

 ing. Whereas growth of some plants is terminated at from io° to i5°C, other 

 plants are still able to develop at o°; thus Soldanella (an alpine plant of the 

 primrose family) begins to develop in the spring when the plants must break 

 through the snow before the shoots reach the air. 



Still more striking variations in minimum and maximum temperatures may 

 be observed in microorganisms. Bacteria are known, for instance, that not 

 only live, but multiply vigorously, at o°C. In sea water at o° have been found 

 as many as 150 bacteria per cubic centimeter. If such water is allowed to stand 

 without change of temperature, this number increases to 1750 in four days, 

 which shows that bacteria continue to reproduce at the temperature of the 

 freezing point of water. 



Bacillus thermophihis is very different from the bacteria just mentioned, 

 being able to reproduce actively at 7o°C. While the optimum temperature 

 for most bacteria lies between 10 and 15 , Bacillus thermophilus ceases to 

 reproduce at temperatures below 42 . 



Bacterial spores can endure great extremes of temperature, some being able 

 to withstand a short period of exposure in liquid oxygen at — 2i3°C. The 

 spores of some soil bacteria can bear very high temperatures, but the higher the 

 temperature is, the shorter is the time required to kill the spores. The time 

 periods required to kill such spores in steam at various high temperatures are 

 given below. 



Temperature of Time Required to 



Steam, Kill, 



deg.C. hours 



100 16 



105-110 2-4 



115 0.5-1.0 



125-130 0.08 



135 o .02-0 . 08 



140 0.02 



Temperature affects the configuration as well as the rate of enlargement of 

 plants. In polar regions and on high mountain-tops, where the temperatures 

 are low, it is usual to find plants very short and lying very close to the soil. It 

 has been observed that the soil of high mountains is relatively much warmer 

 than the air, and plants that remain close to the soil are thus in a warmer en- 

 vironment than would be the case if their stems extended up into the air. 

 Moreover, these low forms are covered in winter with a deep layer of snow, 

 which protects them from freezing. The stems and branches of Pinus humilis 

 do not grow vertically into the air but occupy a horizontal position. Even 

 trunks as much as 20 cm. in diameter, which might quite well support a broad 

 top if they had a vertical position, lie almost horizontal upon the soil surface. 

 So much for the observed facts, but experiments are needed for more definite 

 knowledge. Recently, it has been possible to show that changes in tempera- 



