138 HENRY D. HOOKER, JR 



ature, in the other hot water was poured and frequently renewed. The roots 

 grew toward the partition separating the two compartments. When the water 

 was stained with turnsole-bhie or soot, this reaction was not so pronounced, due 

 as Barthelemy thought to the increased conductivity of the water. He emphasized 

 that these are to be considered phenomena of growth and have nothing to do with 

 a motive faculty. The expei'iments are interesting as being the first recorded ex- 

 amples of thermotropism in roots. 



In 1885 Wortmann^ extended his researches in thermotropism to roots. The ap- 

 paratus used will be described more in detail later. It suffices to state here that 

 the rootlets were placed in moist sawdust in a zinc box heated on one side by six 

 gas flames, and cooled on the other side by running water. The temperatures were 

 measured by three thermometers placed in the sawdust. The two end ones al- 

 though only 5.5 cm. apart could measure as great a difference as between 50° and 8° 

 C. Seedlings of Ervum Lens, Pisum sativum, Zea Mays and Phaseolus midtiflorus 

 were used for experimentation. The seeds were soaked for twenty-four hours in 

 water, and then planted in moist sawdust with the radicles pointing down. When 

 they had grown so that the roots were from 0.5 to 3 cm. long, seedlings of equal 

 length were selected and carefully placed in the sawdust of the zinc box, care being 

 taken to have them exactly vertical. The cotyledons wore all pointed in the same 

 direction with their planes of symmetry parallel. The seeds were aligned with the 

 thermometers, so that the temperature at which reactions occurred could be de- 

 termined as exactly as possible. After each experiment the sawdust was removed 

 from the apparatus and was replaced by some freshly moistened. In order to de- 

 termine if the roots had reacted, they were removed from the sawdust by forceps. 

 As roots that had already bent could not be returned to the sawdust without in- 

 jury, they were discarded, and only such as remained straight were replaced. Some 

 of the roots nutated even in the sawdust. Consequently a careful distinction had 

 to be made between the roots which bent diagonally or to one side and those that 

 bent directh^ to or from the source of heat. Experiments with Ervum Lens led 

 Wortmann to the following conclusions: 



1. Roots of lentile seedlings react to one-sided warming by therniotropic 

 bending. 



2. The bending is positive at lower and negative at higher temperatures, in 

 contradiction to Van Tieghem. 



3. Thermotropism has consequently nothing to do with the growth curve for- 

 mulated by Sachs. 



4. Thermotropic reactions result more quickly at higher temperatures (above 

 40°C.) requiring an hour to an hour and a quarter, than at lower temperatures (in 

 the neighborhood of 12°C.) when they take from 3j to 5'- hours. 



After repeated experiments at various temperatures he found a "'critical" tem- 

 perature where positive thermotropism ceases and negative begins. For Ervum 

 Lens this lies at 27.5°C.; for Pisum sntiinim at Z2° to 33°C., and for Zea Mays at 

 37° to 3S°C. This critical temperature has no connection with the optimum tem- 

 perature. 



Experiments with Pisum sativum, Zea Mays, and Phaseolus midtiflorus led to much 



8 Wortman, J., tjber den Thermotropismus der Wurzeln. Bot. Zeit. 48: 193-200. 

 209-216, 225-235, 1885. 



