§2] 



UPON THE DIRECTION OF GROWTH 



465 



This table shows that the sense of thermotropism is not con- 

 stant for all temperatures, but is positive at the lower tempera- 

 tures, negative at the higher ones, and neutral at a certain 

 intermediate one. Also, just as different species vary in their 

 optimum so also do they vary in the temperature of neutrality. 

 As the optimum for growth 

 is high in maize radicles 

 (34°), and low in the pea 

 radicle (26°. 3), so also is 

 the neutral point. The 

 neutral temperature is thus 

 also probably related to 

 the attunement of the or- 

 ganism and is an advan- 

 tageous response. 



Within the range of 

 positive or of negative 

 turning there is a correla- 

 tion between the temper- 

 ature and the angle of 

 inclination of the organ. 

 Klbrcker has paid par- 

 ticular attention to this fact. His results are summarized in 

 the following table and curves, showing for each species the 

 average inclination at each range of temperature : — 



Fig. 133. — Caloritropic curve of Sinapis alba, 

 showing the relation between the inclination 

 of the radicle and the surrounding tempera- 

 ture. The numbers on the left give the in- 

 clination in degrees ; the horizontal series of 

 numbers are the temperatures to which the 

 plant is subjected, the temperature diminish- 

 ing 4° for every centimeter of departure from 

 the source of heat. (After af Klercker, 

 ■91.) 



TABLE LVI 



The Sense (-|-or— ) and the Average Extent (expressed in Degrees op 

 ANGnLAE Deviation from Vekticality) of Thermotropism at Differ- 

 ent Temperatures 



Pisum sativum | -8.9 | -12.9| -27.2|-38.4|-43.9| 



Sinapis alba I +10.5 | +19.0 | +2.4 | (See Fig. 133.) 



Paba vulgaris | - 4.3 | - 6.5 | -9.8 | -19.1 1 -28.9 | 



°C. . . . 13,14,15,16,17,18,19,20,21, 22,23,24,25,26,2T,28,29,SO,81,32,88,34,85,36,3T,88,39,40,41 



We see here that the angle of inclination (whether -|- or — ) 

 increases regularly as we depart from the temperature of 

 neutrality or that of 0° curvature. 

 2h 



