THERMO TROPISM 1 77 



a pronounced thermotropic irritability is present only in a few .plants, and it 

 is a natural result of the conditions of life of an ordinary plant that it should 

 make use of thermotropic reactions only in a minor degree for purposes of 

 orientation. 



Wortmann 1 observed that seedlings of Lcpidium sativum and Zea 

 Mays, as well as the sporangiophores of Phycomyces, curved towards a hot 

 iron plate emitting dark heat-rays. Steyer 2 has, however, shown that the 

 sporangiophore of Phycomyces has no power of thermotropic reaction, so 

 that the curvatures observed by Wortmann may have been due to accessory 

 causes or were possibly heliotropic in character. Wortmann observed that 

 the seedling-shoot of Zea Mays was positively but that of Lepidium 

 negatively thermotropic, although the latter possesses a stronger heliotropic 

 irritability than the former. Steyer, however, found that both plants were 

 positively thermotropic. 



Wortmann 3 has also investigated the radicles of seedlings by growing 

 them in boxes of sawdust, one side being kept hot, the other cold. The 

 roots of Ervum lens were found to be diathermotropic at 27 C., and 

 similarly those of Pisum sativum did not curve out of a vertical position 

 when at 32 to 33 C. On being placed nearer the hot side, however, the 

 roots curved away from it, but when near the cold side showed a positively 

 thermotropic curvature. According to Klercker 4 , however, some roots 

 only show a negatively thermotropic reaction, whereas a strong positive 

 thermotropism is shown, according to Vochting 5 , by the peduncle of 

 Anemone stellata. 



The smallness of the difference in the temperature of the opposite 

 sides, as well as the fact that either a positive or negative curvature may 

 be produced, suffice to show that they are not due to the more rapid 

 growth of the side exposed to heat. According to Wortmann, decapitated 

 roots show the same reaction, and, since hydrotropic stimuli are only 

 perceived by the root-tip, the curvatures can hardly be due to variations 

 in the amount of moisture on the hot and cold sides. In moist sawdust 

 there can hardly be any appreciable difference in the rate of transpiration 

 from the two sides, whereas when an object is exposed on one side to 

 radiant heat-rays in ordinary air, the resulting differences in the rate of 

 transpiration might be responsible for the tropic stimulation. Apart from 

 this effect, it is not known whether radiated and conducted heat exercise 

 a similar thermotropic action. Hence there is no need at present to adopt 



1 Wortmann, Bot. Ztg., 1883, p. 457. 



2 Steyer, Reizkriimmungen bei Phycomyces nitens, 1901, pp. 10, 20. 



3 Wortmann, Bot. Ztg., 1885, p. 193. 



4 Klercker, Die caloritropischen Erscheinungen bei einigen Keimwurzeln, 1891. (Reprint from 

 Ofversigt af K. Vetenskaps-Akademiens Forhandlingar, Nr. 10.) 



5 Vochting, Jahrb. f. wiss. Bot., 1890, Bd. xxi, p. 269. 



PFEFFEH. Ill fs 



