THERMOTROPISM 479 



heated by the sun at the moment. Anemone nemor6sa and Tulipa silvestris 

 behave in the same manner. 



More exact investigations have been carried out by Wortmann (1885) 

 on roots. When roots are grown in water or moist air and heated on one side 

 no curvatures take place, but when they are grown in sawdust thermotropic 

 curvature was very evident. The response was the result of heat conduction not of 

 radiation. Wortmann placed the sawdust in zinc boxes 6 cm. broad, one longi- 

 tudinal wall of which was heated to about 40° C. by being brought into close 

 proximity to a gas jet, while the other was kept at a temperature of about 

 9° C. by means of running water. The temperature of the sawdust on either 

 side thus showed a difference of about 30°, roughly 5° C. for every cm. of 

 diameter. Every root grown in this medium was thus necessarily warmer on 

 one side than on the other ; further, the roots collectively were also, according 

 to their position, subjected to higher or lower temperatures. The thermotropic 

 curvatures resulting were markedly different, those exposed to higher tempera- 

 tures showing negative curvature, those exposed to lower temperatures positive. 

 This result reminds us of Oltmanns' experiments on Phycomyces where helio- 

 tropic curvature was positive or negative according to light intensity. Just as 

 there a state of indifference was brought about by light of medium intensity, so 

 also we may anticipate a similar condition to arise in the root when it is exposed 

 to medium temperatures. As a matter of fact, Wortmann found that Ervum 

 lens exhibited negative curvature only between 27-5° C. and 50° C, and positive 

 only between 26° C. and 12° C. At 27° C, the critical temperature, the reaction 

 was sometimes positive, sometimes negative, and sometimes there was no 

 reaction whatever. In the case of Pisum the critical temperature was about 

 32°, in Zea mats rather higher, i. e., about 38° C. In other cases (e. g. Phaseolus) 

 negative curvatures only could be induced. 



Additional investigations on roots have been carried out by J. af Klercker 

 (1891), who measured the angle made by the curved root with the vertical 

 after the completion of the reaction. The following results on an average of 28 

 experiments were obtained from Pisum : — 



In these experiments we are dealing only with negative curvatures, and we see at 

 once that the stimulus increases markedly with the temperature ; in like manner 

 we observe, in the case of positive curvatures, an increase of the stimulus 

 as we recede from the critical temperature. This is very well shown by 

 Sinapis alba, where, at 24°-29° C, the angle is only 2-4°, but amounts to 19° 

 when the temperature is I9°-24°C. On the analogy of the heliotropic curve 

 (P- 463) it is very probable that when the temperature falls still further the 

 stimulus will again decrease; e. g., Klercker found that in Sinapis, when the 

 temperature was I4°-I9° C, the angle was only 10-5''. It is very desirable that 

 a complete curve should be obtained for one and the same organ, from the highest 

 to the lowest temperatures, for it is obviously only in this way that the funda- 

 mental facts with reference to thermotropism can be fully established. 



If, in spite of the imperfect nature of our knowledge, we inquire 

 into the cause of the stimulus in thermotropism, we meet with the same 

 differences of opinion as in the case of heliotropism. Van Tieghem (1884), 

 who was the first to draw special attention to thermotropism, put forward 

 a theory, corresponding in all respects to that advanced by De Candolle 

 for heliotropism. He argued from the known facts as to the influence 

 of heat on longitudinal growth (p. 300). When an organ is subjected to a 

 temperature lower than the optimum, the warmer side elongates more vigorously 

 than the other, and the curvature is consequently negative in character ; 

 if, however, the temperature be above the optimum, the cooler side will grow 



