Ploioman — Belations of Plant Growth to Ionization. 129 



Art. XYI. — Certain Relations of Plant Groicth to Ioniza- 

 tion of the Soil ; by Amon B. Plowman. 



In a series of experiments now in progress at the Harvard 

 Botanic G-arden, there have been observed some interesting 

 phenomena in the relations of plants to electricity. The 

 experiments have been of a widely varied nature, dealing with 

 both static and kinetic charges, through a range of potential 

 from 0-5 to 500 volts. Either platinum or high-grade carbon 

 — usually the latter — was used for electrodes, and special care 

 was taken to insure normal conditions of temperature, light, 

 and moisture. 



The experiments were of course checked off by controls, 

 always using at least two different sets of controls in order to 

 avoid accidental errors. In a well-lighted greenhouse it is not 

 a difficult matter to locate the different plants in such a way as 

 to give fairly uniform light to all. For soil-cultures, pots of 

 the same size were used, filled with equal amounts of care- 

 fully prepared soil, and supplied with measured quantities 

 of water. 



The regulation of temperature presented serious difficulties 

 whenever a considerable amount of current was used, for the 

 resistance of the soil is very great and much heat is evolved in 

 forcing the current through it. Thus a 500 volt circuit, 

 joined through a body of soil 10x25^°^ in cross-section and 

 40*^™ long between electrodes, gave a current of 0'2 ampere, and 

 in one hour raised the temperature of the soil from 16° to 1:I°C. 



However, with a slight excess of moisture and a current not 

 exceeding '05 ampere, it has been possible to prevent a rise of 

 temperature of more than 3° above the normal : and in most 

 cases the temperature has been kept within a range of 1° 

 above that of the control. In the matter of temperature- 

 regulation the water-culture method possesses many advan- 

 tages. Either by a slow circulation of the electrolyte, or by a 

 more rapid circulation of water about the vessel containing the 

 electrolyte, it is easy to maintain a constant temperature even 

 when using currents of 2 amperes. In this way it is possible 

 to introduce a very considerable electrical factor without 

 seriously disturbing the other relations of the plants. 



Among other facts recorded in the course of these experi- 

 ments is the following : Seeds placed near the anode are 

 always killed by current amounting to '003 ampere or more, if 

 continued for twenty hours or longer, while seeds placed near 

 the cathode have in most cases been but little affected, and 

 under some conditions have been apparently stimulated by 

 such currents. 



