7IO PLANT RESPONSE 



spontaneous, without recognising or tracing that energy 

 which in the form of stimulus must have been supplied to 

 some part of the plant machinery. 



We have seen that a plant in a state of growth-standstill 

 has its activity renewed when a stimulus is applied to the 

 distant root, and that when the amount thus supplied is 

 exhausted, the activity ceases. In growth, then, which is 

 regarded as so characteristic a phenomenon of vital action, 

 we see the law of the conservation of energy holding 

 good, as in an ordinary inorganic system. The plant thus 

 expresses the absorbed energy, by a responsive expan- 

 sion, either of erection or of increased rate of growth, 

 according to the particular organ of response which is 

 concerned, both of these constituting cases of work done 

 by internal energy, or indirect effect of stimulus ; but where 

 the responding organ is directly excited by external stimulus, 

 we obtain a contractile response of the organ, with expulsion 

 of water, or negative turgidity-variation. The hydraulic cur- 

 rent in our model is now reversed, and opposite responsive 

 movements take place, by the fall of growth below the 

 normal rate, and by depression of the leaf. If now we take 

 a balanced record of growth, the impact of a uniform series 

 of external stimuli will be found to give rise to a series of 

 responses by depression of the rate, followed by recovery, 

 exactly similar to those records of depressions of the leaf, 

 with recoveries, which are obtained from pulvinated organs. 



We may also see the expression of external stimulus and 

 cessation of stimulus, in the appropriate periodic variations 

 of growth, and movements of motile organs, which occur 

 under the stimulating action of daylight, and the withdrawal 

 of such stimulus at night. Thus in the daytime we see 

 a response consisting of depression of the rate of growth, 

 which corresponds to the depression of the leaf, say of Mimosa, 

 and at night a recovery, or enhancement of growth, and 

 erection of the motile organ. The daily periodic curves 

 obtained of this growth-variation and responsive mechanical 

 movement are very similar. 



