250 TLANT RESPONSE 



Channels for conduction of effect of stimulus. -Before 

 concluding this chapter, it is important to consider the channels 

 through which stimulus is conducted with the greatest facility. 

 Since the conduction of stimulus is due to the transmission 

 of protoplasmic change, it is clear, as already said in a 

 previous chapter (p. 60), that such changes will be con- 

 ducted most easily along those paths in which there is least 

 interruption of protoplasmic continuity. It is evident, there- 

 fore, that certain elements in the fibro-vascular bundles will 

 furnish the best conducting medium for the transmission of 

 stimulus. It also follows that in the fibro-vascular tissue 

 itself, the conduction along the length would be more rapid 

 and complete than across. 



On the other hand, the cells of indifferent tissue, such as 

 the parenchyma of the leaf, are divided from each other by 

 more or less complete septa, the fine filaments by which 

 neighbouring cells may be protoplasmically connected being 

 so minute that the conduction of stimulus through such im- 

 perfect channels must be exceedingly feeble. 



These theoretical conclusions I have been able to verify 

 by direct measurement of conductivity in different kinds of 

 tissue. In this investigation, as motile tests of the state of 

 excitability were not available, I devised an electrical method 

 — to be referred to briefly in the next chapter, and described 

 more fully elsewhere— by which to attack the problem. 



Using this method of investigation, I found that plant- 

 organs which contained fibro-vascular elements, such as the 

 stem, peduncle, and petiole, were the best conductors of the 

 state of excitation, and that conduction in such organs is 

 much greater along the length than . across it ; in the 

 peduncle of Mtisa, for example, the conductivity lengthwise 

 is three times as great as that crosswise ; and finally I found 

 that though indifferent tissues like the parenchyma are 

 directly excitable, yet there is practically no transmission of 

 that state of excitation through such tissue. 



From anatomical and other considerations, Dutrochet and 

 Haberlandt came to the conclusion, that it was certain ele- 



