392 PLANT RESPONSE 



that excitation is transmitted upwards by conducting tissues 

 and we have already seen, in studying transmission of excita- 

 tion (p. 250), that the fibro- vascular elements are those which 

 conduct best. It is therefore to be expected that the move- 

 ment of water, which is itself an excitatory effect, should 

 also follow by preference the length of the fibro-vascular 

 elements ; and it is here worthy of note that the ascent of 

 sap is known to take place preferably along such channels. 

 Again, I have shown (p. 250) that the power of conducting 

 excitation is greater along the length of the plant than 

 across ; and we find also that the same is true as regards the 

 transport of water, which is known to be greater in the direc- 

 tion of length. Conduction of excitation takes place with 

 very great slowness across parenchymatous tissues, and the 

 same is the case as regards the conduction of water through 

 such a tissue. If, further, a plant were to be excited from 

 above, instead of below, the transmission of excitation would 

 be downwards, instead of upwards. We might then conceive 

 of the possibility of reversing the direction of the normal 

 transport of water. In such a case, rhythmic excitation would 

 need to be initiated at the top, instead of at the bottom, of the 

 plant. This may be seen in the experiment already referred 

 to, of placing a leafy branch upside down, with its leaves in 

 water. Rhythmic activity due to excessive turgor being 

 now initiated at the upper extremity of the branch, the water- 

 movement is reversed, and sap exudes from the cut end of 

 the stem. Turning back, however, to the subject of the trans- 

 mission of excitation, we have found, it will be remembered, 

 in the case of Biophytum that this takes place with greater 

 rapidity in a centrifugal than in a centripetal direction, that 

 is to say, in the direction of the normal transport of water. 

 It is therefore interesting to note that a reversed water- 

 movement is, in general, known to be somewhat less rapid 

 than the normal flow. 



Rapidity of ascent of sap accounted for by stimula- 

 tory action. — One great difficulty with regard to the ascent 

 of sap has lain in its relatively great rapidity. No theory 



