396 PLANT RESPONSE 



conduction through long tracts, the fibro-vascular tissues are 

 the most favourable. But even in this case the transmission 

 of the excitatory effect may be much enfeebled by distance. 

 Or an interposition of parenchymatous elements may offer a 

 relative obstruction to the transmission. In such cases there 

 might be pseudo-conduction by means of ' relays.' For as an 

 example of the last case we may imagine a mass of excitable 

 parenchymatous tissue, against which a conducting tissue 

 abuts. This mass, being supplied with water by the conduct- 

 ing elements, may become over-turgid, and thus rhythmic 

 activity may be initiated in it de novo. 



That rhythmic activity may under favourable circum- 

 stances be started locally in a mass of excitable tissue, we 

 have seen in the case of the pulvinus of Desmodium. For the 

 fact that activity in this case was not due to any transmitted 

 impulse was proved in the experiment on the localisation of 

 the excitable area, when it was found that an isolated leaflet, 

 if sufficiently turgid, could pulsate. Under natural condi- 

 tions, the necessary turgidity is maintained by the cellular 

 activity of the tissue below. It is worth while to remember, 

 in this regard, that the characteristic pulsation of the leaflet 

 has no immediate connection with that rhythmic activity of 

 the plant-tissue which brings about the ascent of sap. The 

 period of pulsation of the leaflet is determined by certain 

 constants of its cell-complex. We may in fact have various 

 vibration-periods in different organs of the same plant, the 

 different oscillations being brought about by the turgidity 

 caused by the ascent of the sap, just as the same electrical 

 current may give rise to various frequencies of vibration of 

 different electro-magnetic vibrators included in the same 

 circuit. 



Excretion of water. — The rhythmic activity of a mass 

 of excitable cells is seen again in the case of such as ac- 

 tively excrete water. A very striking example is that of 

 Colocasia esculentum, in which the successive expulsions of 

 water-drops noticed by Musset were as many as eighty-five 

 in one minute. 



