INQUIRY INTO CAUSES OF AUTONOMOUS MOVEMENTS 307 



long-continued periodic pulsations, which are kept up during 

 the whole time in which the leaflets are maintained at an 

 unvarying given temperature ? 



We are thus led to see that stimulation is not in all cases 

 dependent upon the occurrence of a sudden change in the 

 exciting cause. On the contrary, excitation may be pro- 

 duced, as we have seen, under a constant and uniform 

 exciting condition. If we analyse the multiple responses 

 produced in Biophytum as the after-effect of the application 

 of a strong thermal stimulus, we find, as said before, that 

 some part of this thermal energy remains latent, and after- 

 wards gives rise to recurring pulsations. 



5. Of internal hydrostatic pressure. — The isolated animal 

 heart, when in a state of standstill, is found to renew its 

 excitatory pulsation under an increase of internal hydrostatic 

 pressure. I shall show later (p. 349) that a Desmodium 

 leaflet, similarly, when in a state of standstill, can be made 

 to resume its excitatory rhythmic activity, by increasing the 

 internal hydrostatic pressure. 



The true meaning of ' tonic ' condition. — We have thus 

 seen that under the continuous action of a constant source of 

 external stimulus, multiple responses are produced. We have 

 also seen that the excess of energy absorbed remains latent 

 in the tissue, in consequence of which, even on the cessation 

 of external stimulation, the pulsatory movements are main- 

 tained for a longer or shorter time. It is thus the excess of 

 stimulus absorbed which renders the tissue excitable, or 

 'tonic' Hence we may have tonicity imparted by light, 

 photo-tonus ; by favourable temperature, thermo-tonus ; by 

 electric current, electro-tonus ; by internal hydrostatic pres- 

 sure, hydro- tonus ; or by the presence of favourable chemical 

 substances, chemo-tonus. W T e have seen that each one of 

 these, by itself, was competent to give rise to multiple 

 responses. It has been shown further that there is no hard- 

 and-fast line between such multiple and automatic responses, 

 the one passing imperceptibly into the other. 



We have now seen that the tonic condition of the plant is 



x 2 



