48o TRANSFORMATION OF ENERGY 



more vigorously and a positive curvature towards the source of heat will ensue. 

 As we may readily understand, the response in the case of the root does not 

 at all correspond to that of the stem, and hence Wortmann (1885) was perfectly 

 right in repudiating Van Tieghem's hypothesis. In some cases, certainly, this 

 theory may explain the facts. Thus Vochting (1888 a) carried out a careful 

 investigation into the mode of unfolding of the buds of Magnolia, where 

 the curvatures are negative owing to the illuminated side growing more rapidly 

 than the shaded. That this is due to heat alone and not to light, has been 

 conclusively proved by Vochting. In all probability, however, this is not a 

 case of thermotropic movement, for in other organs, e.g. fruits, similar movements 

 may be induced, if they be made to grow more rapidly on one side than the other. 

 Heat does not act in this case as a specific stimulus, but as a ' formal condition '. 

 Generally, however, this is not the case, for Wortmann (1885), in some experi- 

 ments, found that those parts of the roots grew most vigorously which were 

 subjected to a temperature, which, operating on all sides of the organ, did 

 not permit of any further growth (supra-maximum temperature). 



After having refuted Van Tieghem's hypothesis, Wortmann assumed that 

 in thermotropism we are dealing with the direction in which the heat rays pene- 

 trate the plant. He has advanced no proof of this, however, and it could only 

 occur in cases where radiant heat was the cause of the thermotropism. As far as 

 regards roots grown in sawdust, we cannot speak of heat radiating in a definite 

 direction since diffusion of heat by conduction is the first consequent ; there is 

 indeed a direction of temperature decrease, bu' not of heat rays. So far as we know, 

 however (compare Vochting, 1888 a), thermotropism due to radiant heat cannot 

 be distinguished from thermotropism due to conduction. We are entitled to 

 assume that the cause of the stimulus is the same in both cases, and that it 

 lies in the dissimilar temperature on opposite sides, a difference which the plant 

 recognizes and to which it reacts. Obviously, growth may express itself on the 

 individual sides quite differently from what it does when all sides are subjected 

 to a uniform and equally high temperature. In heliotropic curvature also we 

 saw that the illuminated side under certain conditions grew more rapidly than 

 it did when all sides were equally illuminated. We are ignorant how great the 

 difference in the temperature affecting the two sides must be before a stimulus 

 is effected, nor has the effect of the height of the absolute temperature on the 

 liminal intensity of the stimulus been determined, nor how the stimulus is in- 

 creased by rise of temperature. In this direction there is ample room for 

 experimental inquiry. 



As to the purely physical or chemical aspect of heat as a stimulus, looked 

 at from the point of view of perception, we know nothing. We have compared 

 thermotropism with heliotropism, but at the same time we do not mean to imply 

 that the perception which precedes curvature is the same in both cases. That 

 goes without saying, for one of the data which Wortmann has established 

 points indeed quite the opposite way ; roots which have their apices removed 

 exhibit thermotropic curvature, and hence the root apex cannot be the organ 

 of perception, or at least cannot be so exclusively, although we must admit it to 

 be so in the case of heliotropism. 



Associated with heat is electricity, which is propagated in waves by radiation 

 or conduction. Although it has recently been shown that radiant light, heat, and 

 electricity are closely related forms of energy, which differ from each other only 

 in wave length, we must not conclude that on that account they operate on the 

 plant in the same way. The variation in the amplitude of the waves, which 

 in the case of light are infinitely small in relation to the plant, but which in 

 the case of electricity markedly exceed the diameter of a seedling or a Phyco- 

 myces, may induce markedly different results in organisms. It is impossible 

 to draw any conclusions in this connexion from the solitary experimental 

 investigations conducted by Hegler (1891), in which he established negative 



