136 HENRY D. HOOKER, JR. 



subjected to a one-sided stimulus of moisture, because of the greater evaporation 

 from the drier side. In order to test this hypothesis, he placed rootlets of Pisum, 

 Sativxim and Vicia Faba so that they received radiated heat from one side. His fail- 

 ure to obtain any reaction sufficed to disprove his theory, as the difference of tem- 

 perature obtained was very small, corresponding to any difference that could be 

 produced by evaporation in the cases in question. It did not however suffice to 

 disprove the existence of thermotropism in roots. 



In the Traite da Botanique,^ a French translation of Sachs' textbook of Botany 

 with numerous annotations. Van Tieghem introduced several passages relating 

 to this subject. He in fact originated the term "thermotropism," which in the 

 sense made use of by him, signifies the ability of any organ to bend in reaction to 

 a one-sided heat-stimulus. He makes the following deductions, which sound hy- 

 })othetical, although he says in the second edition of his work^ that they were 

 substantiated by experimental results. "If the source of radiation is placed lat- 

 erally, so that the plant receives more heat from one side than from the other, its 

 growth will be unsymmetrical and consequently it will bend toward the source of 

 heat or away from it, according to the temperature. If the plant receives the radia- 

 tion on one side at its optimum intensity and on the opposite side at a point signif- 

 icantly higher or lower, it will become convex on the optimum side where it grows 

 more, concave on the other side where it grows less, and it bends away from 

 the optimum. If the two different temperatures are both above or both below the 

 optimum, the organ curves itself in the first case toward the higher; in the second 

 case toward the lower; always toward the one furthest removed from the optimum. 

 It makes no difference if one temperature is above the optimum and the other 

 below, unless they are just so, that the respective rates of growth have the same 

 value, in which case the organ does not bend at all." He says further that .second- 

 ary roots and roots of a higher order are especially sensitive to a one-sided heat- 

 stimulus, because they are free from the hampering effects of geotropism. 



In 1883 experiments were made by J. Wortmann^ which did not allow of Van 

 Tieghem's interpretation, but which led him to the conclusion that in thermotro- 

 pism he was dealing with vital phenomena. As such they must be ranked along 

 with those of geotropism and heliotropism as "a peculiarity of the plant organism 

 based on the structure of irritable protoplasm to react to a one-sided disturbance 

 through known factors or agents by means of bendings produced by growth, which 

 bring the irritated plant part to a position of equilibrium, having a definite rela- 

 tion to the direction of the acting agent." Since these experiments of Wortmann 

 are important as having established this conception of thermotropism in the liter- 

 ature of physiology, and as constituting the main bulk of work on this subject, it 

 will be advisable to discuss them here at some detail. 



The first experiments were made with the sporangiophores of Phycomycef< nitens 

 and with seedlings of Lepidiuvi sativum and Zea Mays. They were carried out in a 



''Van Tieghem, P., Traite de Botanique par Julius Sachs, conforme k I'etat 

 present de la science. Traduit de I'AUemand de la 3ieme edition et annot6. P. 

 116, pp. 301-302. Paris, 1874. 



* VanTieghem, P., Second edition of the same, p. 245. Paris, 1884. 



* Wortmann, J., tJberdenEinfluss der strahlender Warme auf wachsende Pflan- 

 zenteile. Bot. Zeit. 41 : 457-470, 473-480, 1883. 



