390 PLANT RESPONSE 



cellular activity of the plant-tissue. I shall show that this 

 activity is sufficient to account not only for the phenomena 

 of the excretion of solution from pores and nectaries, but also 

 for other phenomena, whose connection with it has been little 

 suspected. 



The mechanics of the ascent of sap. — We have seen 

 that throughout the plant there are active rhythmic tissues, 

 which by their excitatory activity bring about the movement 

 of water. We have next to inquire, therefore, as to how this 

 excitatory action is initiated, and further in what manner so 

 many activities in different zones of the stem are correlated, 

 so as to give rise to a uni-directional flow, generally upwards ; 

 for the rhythmic activity which may cause any given group 

 of cells to act as a pump, would not alone be sufficient to 

 account for the regulated, one-directioned flow of sap, since, 

 while one such group propels water in one direction, there is 

 no obvious reason why another should not propel it in the 

 opposite. 



{a) Uni-directioned flow. — In connection with the trans- 

 mission of multiple excitatory waves, such, for example, as 

 those which we have seen in Biophytum^ it is important to 

 note that these repeated excitations are all initiated at the 

 original point of stimulation, and are propagated in proper 

 sequence from point to point outwards. Thus, after each 

 wave of excitation is exhausted, it starts anew from the 

 original point. This sequence of multiple excitations is 

 exactly parallel to that which is observed in multi ciliated 

 ti.ssues in which the cilia repeatedly contract in sequence. 

 Thus a single cilium at one end gives, as it were, a signal 

 which is followed serially by the rest. 



This being so, it is clear that if such a multi-ciliated 

 tissue take the form of a hollow tube, with the ciliated surface 

 inwards, and if the tube be filled with water, then, owing to 

 this peculiarity of the multiply-responding cilia, the water 

 will always be driven in one direction. A somewhat similar 

 phenomenon occurs in the blood- circulation of animals, where 

 the sinus giving the signal, the rhythmic contraction of the 



