PROPULSION OF SAP AND ITS VARIOUS EFFECTS 391 



heart proceeds towards the ventricle, and the pumping-action 

 thus initiated produces a one-directioned flow of fluid. 



In the leaf of Biopliytum, strongly excited at, say, the 

 inner end, we have similar rhythmic excitations, passing in 

 regular succession from leaflet to leaflet, the innermost leaflet, 

 which is near the seat of multiple excitation, giving the 

 signal to the rest. And just as rigid sequence is observable 

 in the movements of motile cilia, so it is also seen in the 

 plant, by the depression in orderly series of its lateral motile 

 leaflets. 



(fr) Initiation of multiple rhythmic excitations. — We have 

 just seen that in multiple rhythmic response, the multiple 

 waves of excitation proceed serially outwards, from the point 

 of excitation. We also saw in Chapter XXI. that the wave 

 of excitatory contraction is attended by the propulsion of 

 water in the direction of propagation of excitation. It 

 remains to be determined, then, with regard to the ascent of 

 sap in a plant, how this one-directioned propagation of 

 excitation is initiated. 



In the case of an intact plant, the root is acted on con- 

 stantly by various forms of stimulation, among which are 

 (1) contact with the soil ; (2) friction of the growing organ 

 against rough surfaces ; (3) turgor of its own tissues, due to 

 absorption of water ; and there may also be in addition, 

 stimulation by (4), chemical substances of various kinds in 

 the soil, which possibly exert some excitatory influence. All 

 these factors, separately or in combination, serve to set up 

 multiple rhythmic excitation at the extremity of the plant, 

 and the excitatory effect is transmitted upwards, preferably 

 along certain better-conducting tissues. Similarly, in the 

 case of a cut branch placed in water, rhythmic excitation is 

 initiated at the lower end by excessive turgor, just as we 

 found rhythmic movements to be initiated in Desmodium 

 leaflets at standstill, by the artificial increase of internal hydro- 

 static pressure, that is to say, by excessive turgor (p. 348). 



Connection between conduction of excitation and con- 

 duction of sap. — It was said in the case of the intact plant 



