218 



THE RATE OF GROWTH 



[CH. 



The annexed diagram (Fig. 62), showing growth in length of the 

 roots of some common plants at various temperatures, is a sufl&cient 

 illustration of the phenomenon. We see that there is always a 

 certain temperature at which the rate is a maximum ; while on either 

 side of the optimum the rate falls off, after the fashion of the normal 

 curve of error. We see further, from the data given by Sachs and 

 others, that the optimum is very much the same for all the common 

 plants of our own chmate. For these it is somewhere about 26° C. 



80 



70 



M^ie 16 20 22 24 26 28 30 32 34 36 38 40° 



Temp. 



Fig. 62. Relation of rate of growth to temperature in certain 



plants. From Sachs's data. 



(say 77° F.), or about the temperature of a warm summer's day; 

 while it is considerably higher, naturally, in such plants as the melon 

 ot* the maize, which are at home in warmer countries than our own. 

 The bacteria have, in like manner, their various optima, and some- 

 times a high one. The tuberculosis-bacillus, as Koch shewed, only 

 begins to grow at about 28° C, and multipUes most rapidly at 

 37-38°, the body- temperature of its host. 



The setting and ripening of fruit is a phase of growth stiU more 

 dependent on temperature; hence it is a "dehcate test of climate," 

 and a proof of its constancy, that the date-palm grows but bears 



