RESPONSE TO STIMULUS OF LIGHT 



401 



one of the electrodes lies in rise of temperature. Though in 

 all such experiments the incident light should pass to the 

 organ through a thick stratum of water, which absorbs its 

 heat-rays, yet the absorption of light by the tissue must 

 necessarily occasion a slight rise of temperature. In con- 

 nection with this should be remembered the fact I have 

 elsewhere demonstrated, that though sudden variation of 

 temperature acts as an excitatory agent, yet a slow and 

 gradual rise, enhancing the internal energy, brings about 

 only a slight positivity, opposite to the effect of true excita- 

 tion. In this particular ex- 

 periment, however, as the 

 electrical contacts are not 

 directly acted on by light, 

 we obtain results uncom- 

 plicated by such disturbing 

 factors. 



The first electrical effect 

 brought about in the lower 

 half of the pulvinus by 

 application of light on the 

 distal upper half is seen in 

 fig. 237 as an increasing gal- 

 vanometric positivity. This 

 is concomitant to the hydro- 

 positive effect at the lower 

 half, which, conspiring with the contraction of the upper, 

 produces that up-movement of the leaf seen in the previous 

 figure. The excitatory effect next reaches the lower half, 

 and we there obtain increasing galvanometric negativity in 

 consequence. This corresponds with the mechanical move- 

 ment of depression. From this experiment it is clear that 

 light, like other forms of stimulus, induces, as its true excita- 

 tory reaction, galvanometric negativity, the indirect or hydro- 

 positive effect being one of galvanometric positivity. 



In the last case, then, we obtained a transverse trans- 

 mission of the true excitatory effect. Similar effects are 



D D 



Fin. 237. Electrical Response in the 

 Lower Half of the Pulvinus of 

 Mimosa due to Stimulation of 

 Distal Upper Half by Light 



Observe the first phase of positivity, 

 due to hydro-positive effect, con- 

 verted subsequently into negative 

 by the transmission of true ex- 

 citation. 



