REVIEW OF RESPONSE, SIMPLE AND MULTIPLE 717 



most easily along those paths in which there is most proto- 

 plasmic continuity. Parenchymatous tissues, in which the 

 cells are divided from each other by more or less complete 

 septa, are thus relatively inefficient as conductors of the 

 excitatory effect to a distance. Hence, the lamina of the 

 leaf does not transmit its local excitation to any distance ; 

 but tissues which contain fibro-vascular elements, and 

 which are thus characterised by a greater protoplasmic 

 continuity, are therefore better conductors of excitation. 

 Thus stems, petioles, and peduncles are better conductors 

 than the laminae of leaves, and parallel-veined leaves, again, 

 are better in this respect than reticulated. As regards stems, 

 petioles, and peduncles, moreover, the conducting power is 

 greater longitudinally than transversely. In the peduncle 

 of Musa, for example, I find the conductivity lengthwise to 

 be three times as great as that crosswise. In consequence 

 of this difference, it is found that the transmitted excitatory 

 effect of a stimulus unilaterally applied is greater on the 

 same than on the opposite side. This explains the appear- 

 ance of responsive concavity at a distance from the point of 

 stimulation, but on the same side. 



It is to be remembered that, owing to the fact that this 

 conduction of the effect of stimulus is an excitatory process, 

 we find that in autumn and winter, when the physiological 

 excitability is low, the conducting power of the tissue is also 

 very much reduced. 



Various degrees of conductivity are possessed by different 

 tissues, and the distance to which the excitatory effect is 

 conducted depends not only on the conducting power, but 

 also on the strength and duration of stimulus. Thus, while 

 in a feebly conducting tissue the effect of moderate stimu- 

 lation is not transmitted to any distance, strong and long- 

 continued stimulation is transmitted to a certain extent. 

 Even in a better conducting tissue the excitatory effect of a 

 moderate stimulus, on account of gradual enfeeblement, can 

 only reach up to a certain distance. It is to be borne in 

 mind that, while no tissue is absolutely non-conducting, 



