466 EFFECT OF HEAT UPOIf GROWTH [Ch. XVIU 



3. Causes of Thermotropism. — Van Tieghem suggested, in 

 his a priori account of thermotropism, that it was due to an 

 unequal growth on the two unequally heated sides of the 

 organ, the side whose temperature was nearest the optimum 

 making the greater growth. This explanation is not a 

 direct mechanical one, the turning stem does not act merely- 

 like a metallic rod. The curvature is rather the result of 

 unequal growth at unequal temperatures. In accordance with 

 the theory just outlined we should find organs subjected on 

 one side to the optimum temperature growing on that side 

 faster than on the other, and hence turning from the optimum 

 — but they turn towards it. Again, plants subjected on one 

 side to a temperature slightly below the optimum will have a 

 still lower temperature on the opposite side, and should be 

 negatively thermotropic — but they are positive. Finally, 

 plants subjected on one side to a temperature above the opti- 

 mum will have a lower temperature, one nearer the optimum, 

 on the opposite side, and should be positively thermotropic — 

 but they are negative. In a word, according to the theory, 

 plants should turn from the optimum ; they turn, however, 

 towards the optimum, hence van Tibghem's theory is exactly 

 opposed to the facts. 



Woetmann, on the other hand, believed that the sense of 

 thermotropism is due to a self-regulation of growth in the 

 organism leading it to make this advantageous turning. The 

 stem tends to place itself in the axis of the heat rays just as in 

 phototropism it places itself in the axis of the rays of light. 

 To this theory we must, however, add that plant organs may 

 respond to conducted heat as well as to radiant heat, since 

 they turn in a direction which, although opposed to the ordi- 

 nary law of growth, tends to bring the tip into its optimum 

 temperature. Such a result indicates clearly that thermotro- 

 pism is a response to stimulus. 



If thermotropism is a response, where is the perceptive 

 region? Suspecting that it was at the apex, Wortmann de- 

 capitated the root tip, but the response occurred as before. 

 Evidently the whole growing part is sensitive to temperature. 



Thermotropism is thus seen to be such a response to the stimu- 

 lus of either radiant or conducted heat that the organ — plumule, 



