272 THE LIFE OF THE PLANT 



while their temperature noticeably rises. This pheno- 

 menon is manifested by all parts of the plant during 

 its whole lifetime, but with the difference that the 

 green parts also decompose carbonic acid and give off 

 oxygen much more energetically under the action of 

 light, so that this process conceals the respiration 

 taking place simultaneously with it. 1 But is respira- 

 tion connected with motion ? Experiments answer 

 in the affirmative, although the nature of this connexion 

 is not as yet clear to us. If we arrest the supply of 

 oxygen to the plant, all the phenomena of movement 

 will cease accordingly : the protoplasm will stop flow- 

 ing, the stamens of the barberry, the leaves of the 

 mimosa, will lose their irritability, and only after being 

 placed for a considerable time in an atmosphere con- 

 taining oxygen will these phenomena recur. Conse- 

 quently the movements of plants as well as the move- 

 ments of animals are closely connected with respiration. 

 Let us proceed with our comparison. Muscles 

 become heated by contraction ; their temperature rises 

 to a measurable though insignificant degree : the same 

 is to be observed in plants. By applying to the pulvinus 

 of the leaf-stalk of mimosa a very sensitive thermometer 

 called a thermopile, it has been found possible to show 

 that the temperature rises at the moment of motion. In 

 muscles at rest, as also in a state of tension, the presence 

 of an electric current is noticed. If a circuit connected 

 with a sensitive galvanometer is closed (with all due 

 precautions) by means of a muscle of a frog, the needle 

 of the galvanometer will swing and manifest the current 

 in the circuit. A similar though weaker current will 

 become apparent, if instead of a muscle, a leaf of the 

 catchfly is introduced into the circuit. The similarity 

 does not stop there. If the muscle is made to contract, 

 a slight' decrease in the strength of the current is ob- 

 served at the moment of contraction, the needle of the 



1 See chapter v. 



