THERMOTROPISM IN ROOTS 151 



CONCLUSIONS 



Klercker'^ mentioned five causes as possible factors in the ther- 

 motropism of roots as described by Wortmann but termed by 

 himself caloritropism. To these we will add one, which gives the 

 following list: (1) geotropism, (2) radiated heat, (3) air currents, 

 (4) conducted heat, (5) hydrotropism, (6) traumatropism, 



Geotropism cannot be considered an initial cause since it 

 merely modifies the subsequent developments as Klercker has 

 described. It therefore need not be considered here. 



It seems improbable that radiated heat has an effect on roots. 

 The experiments made with roots suspended in moist air subject 

 to a difference of temperature demonstrate this. Similar experi- 

 ments were made by Wortmann'"^ with the same results. The 

 same objections may be raised against air currents as determining 

 factors. 



The phenomena under discussion have till now been attributed 

 to conducted heat. The experiments with the agar-agar conclus- 

 ively disproves this. It is essential for the validity of these re- 

 sults that the roots grow and be capable of reaction in the solution. 

 Tests were made with geotropism, and the roots were observed to 

 react without difficulty. According to Porodko, the normal rate of 

 growth is one millimeter per hour. The tabulated data of the ex- 

 periments show that the roots actually grew and that the rate of 

 growth is from one half to two thirds the normal. On this account 

 the reaction would be expected to require more time. In all ex- 

 periments, the roots were therefore exposed to the thermotropic 

 stimulus for a period of more than three times that ordinarily 

 necessary, usually from seven to eight hours. After twelve hours 

 in the agar, the rate of growth materially diminishes, in all proba- 

 bility because of more difficult respiration. This did not interfere 

 with the experiments, however, which were finished before this 

 time had elapsed. 



The actual difference between the temperatures on the two 

 sides of a root is exceedingl}' small in Wortmann's apparatus. 



'^ Klercker, J. af, loc. cit. 

 Wortmann, J., loc. cit. 1885. 



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THE PLANT WORLD, VOL. 17, NO. 5, 1914 



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