26 RESEARCHES ON IRRITABILITY OF PLANTS 



thus be applied. By means of a variable resistance included 

 in the circuit, the intensity of the stimulus can be increased or 

 diminished. Care must, however, be taken that the heat 

 produced in the platinum loop shall not be such as to scorch 

 or otherwise injure the tissue. 



I find that injury from scorching may be avoided by 

 adding a drop of water at the point of contact and after- 

 wards removing excess of water by blotting-paper. This 

 thin film of water protects the tissue from a burn. It is, 

 again, not absolutely necessary to place the platinum wire 



in contact with the plant. 

 Excitation will take place 

 if the heating-wire is in 

 close proximity. How 

 practicable this form of 

 stimulus may be rendered 

 will be observed from the 

 record (fig. 7) of two suc- 

 cessive excitations by this 

 method, which are seen to 

 be uniform. For certain 



Fig. 7. — Response records of Mimosa . . . . . 



under indirect thermal stimulation. electrical investigations it 



is essential that stimulus 

 other than electrical should be employed. This require- 

 ment is admirably fulfilled by the thermal mode of stimu- 

 lation. 



Another mode of stimulation — namely, that of thermal 

 radiation — can also be employed, though not so conveniently 

 as the former. A certain area may be rendered radiant by the 

 passage of an electrical current. A Nernst electrical lamp 

 can be conveniently utilised for the purpose. This, when 

 rendered incandescent, gives out radiation of constant 

 intensity. This radiation consists not only of light rays 

 but also of a large proportion of obscure heat-rays. The 

 excitatory value of the latter is more efficient than the 

 luminous rays. The radiant surface of the Nernst lamp 

 is suitably placed in front of a concave metal mirror, by 



