400 PHYSIOLOGY OF GROWTH. 



temperature readings must be noted, so that we may be able to 

 calculate the mean temperature. 



Each experiment should extend over forty-eight to seventy-two 

 hours, or even longer. It begins the moment the seeds are soaked. 

 At the end of the experiment we take the seedlings out of the soil 

 measure the length of their roots, and so obtain numbers from which 

 we can easily deduce a mean value for the length which a root 

 attains in a particular time at a particular temperature. Thus, 

 e.g., at 25 C., the main root of Zea Mais may attain a length of 

 30mm. in the course of forty-eight hours. At 34 C., a root more 

 than 50 mm. in length may be produced in forty-eight hours, 

 while a temperature of 42 C. considerably impairs the growth of 

 the root of Zea. At 15 C., also, the root grows but slowly, and, 

 therefore, even in ninety-six hours, attains no considerable length. 

 The growth of the plant- cells begins at a certain lower limit of 

 temperature (the temperature minimum), increases in rapidity up 

 to a certain point (the optimum temperature), beyond which the 

 rate of growth again diminishes. The highest temperature at 

 which we find growth to take place at all we term the maximum 

 temperature. The positions of the minima, optima, and maxima 

 of temperature are by no means the same for different structures, 

 which also finds expression, e.g. as regards the minimum tempera- 

 ture, in the readily determined fact that many seeds are still 

 unable to germinate at all at a temperature at which the germina- 

 tion of other seeds is possible. At 10 C. the seeds of Cucurbita 

 do not germinate even after a considerable time has elapsed, 

 whereas wheat grains and seeds of Phaseolus multiflorus do 

 germinate at this temperature. 1 



In the following table is given the position of the cardinal points 

 of temperature for germination of some seeds : 



Temperature : 



Minimum. Optimum. Maximum. 

 Triticum vulgare ... 5'0* ... 287 ... 42'5 

 Phaseolus multiflorus 9'5 ... 33'7 ... 46'2 

 Pisum sativum ... 6'7 ... 26'6 



Zea Mais 9-5 ... 33'7 ... 46'2 



Cucurbita Pepo ... 137 ... 33'7 46'2 



The temperatures are expressed in degrees Centigrade. 



* According to recent investigations, the temperature minimum is usually 

 lower. 



