PHYSIOLOGICAL RESPONSE IN PLANT AND ANIMAL 75 1 



the other hand, we have seen that under its action in nature, 

 heterogeneity is evolved out of homogeneity. Thus it is the 

 unequal action of an external force which, for example, 

 causes a radial organ to become anisotropic, with corre- 

 sponding physiological complexity, culminating in dorsi- 

 ventrality. One instance of an organ in which, owing to 

 this differentiation, a movement of apparent advantage to the 

 plant has been induced, is found in the pulvinus of Oxalis. 

 Here, by the greater excitability of the lower half, the leaf- 

 lets are made to fold downwards, with the consequence of 

 avoiding too intense illumination, in the responsive move- 

 ment known as diurnal sleep. But the differentiation which 

 we find here is not unique or suddenly evolved, for we 

 find a similar anisotropy even in the pulvinoids of ordinary 

 leaves such as those of Artocarpus. And such physiological 

 differentiation can be traced still further back, to the case of 

 organs which were originally radial. Thus, a long stem, 

 such as that of Cucurbita, happening to become recumbent, 

 becomes also dorsi-ventral, by the unequal action of sunlight 

 on the two sides, the too long excited upper side being now 

 the relatively less excitable. There is again no fixed line of 

 demarcation between this more or less permanent and a 

 transient differentiation ; for when a radial stem of Cucur- 

 bita is acted on by a transient unilateral stimulus, a tempo- 

 rary anisotropy is induced as between the excited and the 

 unexcited sides, the latter, which is fresh, being now the 

 more excitable ; but this anisotropy immediately disappears 

 on the recovery from excitation of the excited side. We 

 may thus have in the same organ at different times a 

 transient anisotropy, lasting for a minute or so, under 

 moderate unilateral stimulation ; a more prolonged aniso- 

 tropy, lasting for an hour or so, under stronger stimulation ; 

 and a permanent differentiation under still stronger and 

 longer continued unilateral stimulation. The difference 

 between the first and the last of these is simply a question 

 of whether the limit of elasticity has been exceeded or not. 

 In a torsioned wire, similarly, on the cessation of moderate 



