308 PLANT RESPONSE 



determined by the sum total of the latent energy derived 

 from all the above-named factors. This internal factor, 

 of latent energy, will be shown to play a very important 

 part in all response-phenomena, and as it thus becomes 

 necessary to have some convenient means of referring to it, 

 we shall henceforth designate it as the Internal Energy of 

 the plant. 



Automatic movements in plants are thus only exhibited 

 under favourable tonic conditions. It has been shown that 

 the plant displays rhythmic activity when subjected to different 

 forms of constant stimulus, and we have now investigated 

 separately the effects of such constant stimuli — chemical, 

 electrical, thermal, photic, and hydrostatic. As has been 

 explained before, if a given stimulus be not of sufficient 

 intensity to evoke visible response, yet the absorbed energy 

 may render the tissue capable of responding to another 

 subsequent stimulus, which by itself would have been in- 

 effectual ; in other words, the tissue is made excitable by the 

 presence of a stimulus which has not of itself been adequate 

 to cause response. When this latent excitability exceeds a 

 certain amount, any further increase may be expressed in a 

 visible manner by mechanical pulsation. Now, taking the 

 various forms of stimulus to which the plant is constantly 

 exposed — namely, warmth, light, moisture, and the action of 

 the various chemical reagents, organic and inorganic, present 

 in it or absorbed by it — it is clear that each of these exerts its 

 stimulating effect independently ; any one by itself may then 

 make the tissue excitable to the verge of response ; in this 

 condition, though there is no outward sign of the fact, there 

 is a considerable amount of latent energy ; and the incidence 

 of any second and additional form of stimulation is now 

 sufficient to precipitate the excitation in visible form. These 

 considerations will show how, by the cumulative and additive 

 effects of all the forms of constant stimulation mentioned 

 above, the plant may become so highly excitable as to 

 manifest the fact by giving rise to responses which appear as 

 automatic. The tonic condition is thus the latent excitatory 



