SEPARATE LOCALIZATION OF PERCEPTION AND RESPONSE 195 



When one considers that the power of tropic reaction has been 

 developed for the purpose of bringing the various organs of the plant into 

 different positions suitable to the performance of their special functions, 

 it is evident that the organs will not only have dissimilar irritabilities but 

 also that the area over which a stimulus may spread must be restricted. 

 Otherwise the tropic stimulation of a stem might spread to the root 

 and cause it to perform unsuitable curvatures. In general the purpose 

 of tropic curvature can be attained when the perceptive and active 

 zones are not separated. Hence it is only in special cases that any such 

 separation is shown, or that a pronounced power of transmitting tropic 

 stimuli is developed. The special heliotropic irritability of the apical 

 parts of various seedlings may be of use in rendering possible a curvature 

 towards the light as soon as the tip emerges above ground, the stimulus 

 spreading to and stimulating the parts below the ground. Similarly, it 

 is evidently a purposeful adaptation which leads to the tip of the root 

 receiving geotropic stimuli and regulating the growth of the region behind 

 so that it assumes a proper position. The importance of such localization 

 must, however, not be overestimated, since equally rapid and appropriate 

 orientation is possible when the power of perception is evenly distributed 

 over the whole of the active zone. Teleological considerations must, indeed, 

 never be pressed too far, and they would lead us to conclude that the move- 

 ment of the leaf-stalk into a phototropic position would be best induced 

 by the directive action of the lamina. As a matter of fact, the heliotropic 

 sensibility appears never to be restricted to the lamina, and its orientation 

 seems always to be due to the co-operation of a variety of factors. 



The power of transmitting tropic stimuli across small distances which 

 may surpass the breadth of the organ affected must always be present, 

 for all the cells are not equally irritable, and yet growth activities must 

 be excited in the responsive tissues corresponding to the extent of the 

 induced curvature. In the case of dorsiventral tendrils in which the convex 

 surface is not directly excitable, the stimulus to increased growth must 

 be transmitted from the concave to the convex surface, and probably 

 the same applies to tendrils in general, since it is always the outer side 

 not in contact whose growth in length is accelerated. In addition, Mucor 

 and Caulerpa afford instances in which the different parts of a cell are 

 endowed with dissimilar tropic irritabilities, and Steyer 1 has shown that 

 in the case of Phycomyces the heliotropic sensitivity is restricted to the 

 apex of the sporangiophore. Hence localized unilateral illumination 

 beneath the growing zone produces no heliotropic reaction, either because 

 this zone has no power of perception or because it is unable to transmit 

 the stimulus to the growing zone and so direct the growth of the latter. 



1 K. Steyer, Reizkriimmungen bei Phycomyces 1901, p. 6. 



O 2 



