STONE: CONTACT STIMULATION 455 
touched those of other plants. An electrically charged atmosphere, 
however, exerts a marked stimulation on plants and it is possible to 
modify the function of organisms located at more or less high eleva- 
tions by the use of metal coverings. The old idea that milk sours 
more rapidly during thunder storms, and that plant growth is greater 
following electrical storms has in reality a fundamental basis. The 
discovery of contact stimulation led us to modify our methods of 
studying the effect of electrical potential on plants since we found 
that when the plants were not in contact with one another, or with 
the surrounding wire mesh quite different results were obtained. 
The observations and results obtained by contact of plants with one 
another and with wires, etc., was so significant that we undertook the 
investigation of this phase of the problem at that time, and have 
devoted considerable attention to it since. Some of our earlier experi- 
ments were conducted out-of-doors and parallel experiments were 
carried on in a conservatory. Repeated tests of the air in our con- 
servatory with a water-drip apparatus and electrometer have in- 
variably shown that under ordinary weather conditions there exists 
no atmospheric electricity in conservatories, the glass apparently 
acting as a screen. The nature of the stimulation due to contact is 
probably in no way associated with atmospheric electrical phenomena, 
or at any rate, the growth responses do not appear to be identical 
with those resulting from ordinary electrical stimulation. The 
response to contact is induced by the use of various materials, such as 
wire, twine, wood and metal stakes, excelsior, sphagnum moss, soil 
particles or even by the plants being in contact with one another. 
The same reactions are produced whether the different contact 
materials used are suspended and only touch the leaves of the plant, 
or whether they touch both the leaves and soil in which the plants 
are growing. While there is no evidence to show that these reactions 
are associated with any changes in the electrical tension of the at- 
mosphere, surrounding the plants, they may, however, be connected 
with electrical phenomenon. The reactions resulting from contact 
stimulations are not unlikely quite primitive and universal to plants 
and probably common to the lower forms of life in general. Probably 
all organs will prove to be sensitive to contact but from our observa- 
tion the leaves appear to be especially so. The nature of the reactions 
appear to be fundamentally similar to those of touch, from which it 
would seem the more highly differentiated reactions of tendrils and 
wound responses, etc., originated. 
