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 grtater 

 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. 



