THERMOTROPISM IN ROOTS 



HENRY D. HOOKER, JR. 



Osborn Botanical Laboratory, Yale University, New Haven, Connecticut 



The effects produced by the apphcation of heat to one side of a 

 plant organ, which are termed thermotropic, have received but 

 Uttle attention in recent years, and the thermotropism of roots in 

 particular has not been investigated in the last two decades. Stey- 

 er's paper on Phycomyces nitens^ demonstrated the error of some 

 current opinions concerning thermotropism, and indicated the 

 necessity of further critical work in this field. It is moreover es- 

 sential to check the results obtained by the physiologists of the 

 19th century, by the improved modern methods of work. The 

 subject of this paper was suggested by Professor Jost and the ex- 

 periments were carried out under his and Professor Kniep's direc- 

 tion at the Botanical Laboratory of the University of Strassburg 

 in the winter semester 1912-1913. I take pleasure in thanking 

 Professors Jost and Kniep here for their guidance and generous 

 assistance in helping me with the experimental part of the work. 



HISTORICAL 



The physiological effects of temperature upon plants were recognized early by 

 Sachs. ^ He found that heat had a marked and definite influence on the rate of 

 growth, so that on this basis a minimum, optimum and maximum temperature 

 could be determined empirically for each individual species. By measuring tem- 

 peratures on the abscissae and the rate of growth on the ordinates, this influence 

 may be graphically represented. The resulting curve outlines a bell, and resem- 

 bles the probability curve. 



Sachs^ endeavored to explain hydrotropic bending of roots by the difference 

 of temperature, that necessarily must exist between the opposite sides of a root, 



^ Steyer, K., Reizkriimmungen bei Phycomyces nitenx. Pp. 10-14, 20-21, Leip- 

 zig, 1901. 



- Sachs, J., Jahrb. wiss. Bot. 2: 338, 1860. 



^ Sachs, J., Ablenkung der Wurzeln von ihrer normalon Wachstumsrichtung 

 durch feuchte Korper. Arb. d. bot. Inst. Wiirzburg, 1872. 



135 



THE PLANT WORLD, VOL. 17, XO. 5, 19U 



