18 RESPONSE IN THE LIVING AND NON-LIVING 
It must however be remembered that the response, being 
a function of physiological activity of the plant, is liable 
to undergo changes at different seasons of the year. 
Each plant has its particular season of maximum 
responsiveness. The leaf-stalk of horse-chestnut, for 
example, exhibits fairly strong response in spring and 
summer, but on the approach of autumn it undergoes 
diminution. I give here a list of specimens which will 
be found to exhibit fairly good response : 
Root.—Carrot (Daucus Carota), radish (Raphanus 
sativus). 
Stem.-—Geranium (Pelargonium), vine (Vitis vinifera). 
Leaf-stalk.—Horse-chestnut (dsculus Hippocas- 
tanum), turnip (Brassica Napus), cauliflower (Srassica 
oleracea), celery (Apiwm graveolens), Eucharis lily 
(Hucharis amazonica). 
Flower-stalk.— Arum lily (Richardia africana). 
Fruit.— Eege-plant (Solanwm Melongena). 
Negative variation.—Taking the leaf-stalk of turnip 
we kill an area on its surface, say B, by the application 
of a few drops of strong potash, the area at A being 
left uninjured. A current is now observed to flow, in 
the stalk, from the injured B to the uninjured A, as 
was found to be the case in the animal tissue. The 
potential difference depends on the condition of the 
plant, and the season in which it may have been 
gathered. In the experiment here described (fig. 6, a) 
its value was ‘13 volt. 
A sharp tap was now given to the stalk, and 
a sudden diminution, or negative variation, of cur- 
rent occurred, the resting potential difference being 
