320 VEGETABLE PHYSIOLOGY 



CHAPTEK XXI 



TEMPERATURE AND ITS CONDITIONS 



THE various processes which are characteristic of vegetable 

 life only take place so long as the plant is exposed to a 

 particular range of temperature, which lies between the 

 freezing point of water and about 50 C., a few exceptions 

 on both sides of that range however being met with. It 

 is consequently essential to the well-being of the organism 

 that its temperature shall be maintained within those 

 limits. While life is possible within this range it is not 

 equally well manifested at all the points which lie between 

 the limits ; each vital function indeed shows considerable 

 variation in this respect. There is a certain point, lying 

 generally near the freezing point, below which it cannot be 

 observed. There is another point near the upper limit, 

 beyond which it is not carried out, and somewhere between 

 them there is a point at which it is manifested most 

 advantageously. These three points are known respectively 

 as the minimum, maximum, and optimum temperatures for 

 that function. These temperatures vary for each function 

 which accompanies the life of any particular plant. They 

 are not, moreover, in the case of a particular function, 

 necessarily identical in different plants. 



The process of photosynthesis, for instance, commences 

 in the grasses at about 2 C., while in the Potamogetons it 

 cannot be detected below 10 C. The absorption of water 

 by the roots of the Turnip and other cruciferous plants may 

 begin when the soil has a temperature but slightly above 

 the freezing point of water ; in the case of the Tobacco- 

 plant it must be at 12 C at least. The lowest temperature 



