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CHAPTER XX 
TEMPERATURE AND ITS CONDITIONS 
THE various processes which are characteristic of vegetable 
life only take place so long as the plant is exposed toa 
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 
