TEMPERATURE 95 
acids are present in the free form but some of them 
appear mostly as the acid salts of yarious metals. 
143. Temperature. The relation of the plant to 
temperature will be discussed here as it is chiefly a ques- 
tion of the effect of temperature upon the nutritive 
functions. Five cardinal points for temperature can be 
distinguished for these different processes. They are: 
death point from cold, death point from heat (points 
which are the same whatever the process and mentioned 
here simply because when reached the process cannot 
be resumed when normal temperatures are again re- 
gained), minimum, optimum and maximum. The last 
three are quite different for different life processes. 
Thus the optimum and maximum for respiration are 
usually much higher than for photosynthesis, in fact 
they often lie close to the death point from heat. Be- 
tween the death point from cold and the minimum for 
various processes may be a small range or sometimes 
a great range of temperature. Usually the minimum 
point is a little above or not much below 0° C. The 
maximum temperature for the various functions lies 
usually between 36° and 43° C. and the death point be- 
tween 50° and 55° C., but in a few plants of hot springs 
as well as some bacteria causing the heating of manure, 
etc., the optimum temperature may be about 60° and 
the death point even as high as 75° to 85° C. 
144, The death of plants by heat appears to be due 
to the coagulation of some of the protein constituents of 
the protoplasm. Since this coagulation cannot occur 
unless a certain amount of water is present we find that 
some nearly water-free structures are able to endure 
rather high temperatures. Thus the spores of some 
bacteria can be boiled for several hours before they are 
killed and some seeds can endure a dry heat exceeding 
