140 HENRY D. HOOKER, JR. 



to be produced by growth, only the last concerns us. By protecting the blossoms 

 from the light but allowing the heat rays to have effect, the following of the sun's 

 course, which continued in darkness, was shown to be of ttiermotropic nature. 



The work of John af Klercker" which appeared the following year, was more im- 

 portant. He considered that Van Tieghem's conclusions, which he characterizes 

 as a "theory deduced from purely theoretical considerations," were decisively 

 refuted by Wortmann's results. Special stress was laid on the discovery that the 

 critical temperature was invariably found to be above the optimum. He distin- 

 guished between the experiments with Phyc.omycea niiens, Lepidium sativum and 

 Zea Mays, and those with roots. The first were compared with results obtained 

 by ultra-red rays, and as such was considered as an extreme case of heliotropism. 

 He pointed out that but two factors could be responsible for the bending : first, an 

 immediate stimulus from the heat waves; and second, a temperature difference at 

 the opposite sides of the plant organ. He agreed with Wortmann in attributing 

 the bending to the first of these, but laid little value on his results. The measure- 

 ment of the temperature with ordinary thermometers suspended in the air was 

 criticized, as the rapidity of evaporation, which must vary with the temperature, 

 was left out of consideration. Klercker pointed out that the experiments with 

 roots were carried out in a heterogeneous mixture of sawdust, air and water, pres- 

 ent as vapour in the air and as films of liquid about the grains of sawdust. This 

 rendered possible the joint action of several factors. The following were tabulated 

 as having possible influence: (1) conducted heat; (2) radiated heat from the saw- 

 dust particles; (3) direct stimulus from air currents; (4) hydrotropism; and (5) geo- 

 tropism. He suggested that the term "caloritropism' be used when the heat was 

 conducted, and that thermotropism be reserved for radiated heat. 



Klercker made use of two forms of apparatus. The one was modified from Wort- 

 mann's, the heat being derived from circulating hot water in place of the flames. He 

 also constructed an apparatus of his own. This consisted of three zinc tanks, one 

 within the other. Between the first and the second cold water circulated ; between 

 the second and the third was placed the sawdust for the roots, and in the innermost 

 hot water was kept at a constant temperature through connection with the heating 

 apparatus, that kept the hot water circulating. The sawdust was covered with 

 cardboard, provided with rows of holes for the thermometers. "Qy these, the tem- 

 perature was shown not to vary more than three degrees in seven hours. The roots 

 to be examined were removed with pincers and the angle of inclination was meas- 

 ured against a glass plate. 



Pisutn sativum, Vicia Faba, Helianthus annuus and Sinapis alba were the species 

 experimented with. The results were graphically represented, by measuring the 

 temperature on the ordinates and the angle of inclination from the vertical on the 

 abscissae. Roots of the first three species showed only a negative thermotropism, the 

 angle increasing as a rule with the temperature. Sinapis alba reacted positively. The 

 reaction was especially vigorous between 19° and 24°C., and became weaker above 

 and below this temperature. The critical temperature for Pisum was not deter- 

 mined, but from the results obtained it would be placed at a much lower tempera- 

 ture than Wortmann found for it. From the four widely different curves obtained , 



'1 Klercker, J. af, tJber calotropische Erscheinungen bci einigen Keimwurzeln. 

 Ofvers. Vetensk. Akad. Forhandl. 48: 778, 1891. 



