470 TRANSFORMATION OF ENERGY 



by its reaction) is not less than that of the apex, but its power of perception 

 must be less well developed. When we speak of a more limited sensitivity, we 

 mean the antecedent perception and not the excitation, still less the movement, 

 but, generally speaking, the greater the sensitivity the greater the excitation, 

 although movement, as we have seen, may fail to make itself evident in spite 

 of a very great excitation. 



From among the other examples which are known as supporting the 

 hypothesis of a distinction between perception and reaction in heliotropic 

 phenomena, or at least as confirming the idea of unequal excitation and the 

 transmission of excitation to other regions, we will select the case of the leaves 

 of the Malvaceae, whose behaviour has been carefully observed and described 

 by VOCHTING (1888). We have to deal with, in this case, a surface which, 

 generally speaking, exhibits no active movements, and we have also an articula- 

 tion immediately adjacent which may bring about curvatures by alterations in 

 turgor, and finally a petiole showing growth movements. Necessities of space 

 compel us to limit ourselves to the consideration of the movements of the 



articulation. Such movements may be in- 

 duced in it directly, or they may result from an 

 excitation transmitted to it from the lamina. 

 If unilateral illumination affects the 

 pulvinus only, it behaves almost like a 

 . positively heliotropic stem, and when it 



\ y, bends it gives thereby a different inclination 



to the leaf-blade. The leaf surface, how- 

 ever, so influences the pulvinus that it 

 places the blade at right angles to the 

 * MM. incident rays. When VOCHTING succeeded 



Fig 150. ^ Arrangement for partial darken- by appropriate experimental means in bring- 

 ing of a seedling of Avena. The arrows - J Y\ * i 1,1 TIT-, 

 indicate the direction of the light. II and Ulg the pulviUUS and the blade UltO OppOSl- 

 /// show the result after 5 hours. The finn VIA fnnnH fhat f VIA rmlvinnc in /VhAHiAnr** 

 horizontal line indicates the limits of the tlOI1 ne 1OUna tnat tne P UlvmuS > Ul ODCQience 

 regions subjected to the influence of the ravs tO the impulse transmitted to it frOHl the 



^S^SSni^SSebSr/S^ ! amina responded much more readily than 



seedling has curved to the right in its upper it did to 3. direct Stimulus, and the direct 

 part, and to the left at its base. After , . , 1,1 1-11 



ROTHERT (1894, pp. 17 and 59). stimulus was completely neutralized by a con- 



trary stimulus transmitted to it from the 



blade. The perceptive power of the blade in other cases also is doubtless 

 responsible for the movements of the petiole. 



Further, proof has been adduced by KOHL (1894) in support of the view- 

 that the negative heliotropism of roots arises from a power of perception 

 located in the root apex. As a result of our studies in the localization of helio- 

 tropic perception in the apices of grass cotyledons and of roots, we might 

 be inclined to conclude that geotropic perception is also localized, but such 

 a conclusion drawn from analogy must be rejected, first because geotropism 

 and heliotropism are very different phenomena looked at from a physiological 

 standpoint, and secondly because, from the biological point of view, heliotropic 

 perception, in the apex of the cotyledon, in the apex of the root, and in the 

 surface of the leaf-blade, appears to be entirely purposeful, while the object of 

 localizing geotropic perception in the root apex is not apparent on the face of it. 



Neither is it true that a definite organ is constructed in all cases to perceive 

 the light stimulus, nor is the power of perception less intense at the region of 

 the movement than elsewhere, for there are plant organs which exhibit as great 

 powers of perception in the motile zone as in other regions, and yet in their 

 case also the heliotropic excitation can be transmitted (ROTHERT, 1894). 



Having now studied sufncientlyin detail thebehaviour of the grass seedling, 

 and having, in the course of our study, seen that a heliotropic curvature must 



