INTRODUCTION 53 



Growth under certain conditions may be stimulated 

 by the application of electricity to cultural methods. 

 This was done by Berthollon as long ago as 1783, 

 and the method has lately been perfected by Priestley, 

 Lodge, and others. 



Heat in a similar manner applied artificially at 

 certain stages may promote growth. 



Apart from the nutrient solutions or solutes, and 

 the provision of water and carbon dioxide in the 

 atmosphere, other factors are necessary for plant 

 growth and the manufacture of carbohydrates, 

 proteids, etc. 



These are a requisite supply of oxygen, light, 

 temperature, water, and food, the last of which 

 has been dealt with. As the bearing of the other 

 four factors upon plant growth has been dealt 

 with already, they are only briefly mentioned at this 

 stage. 



Oxygen is required for the parts of the plants both 

 above and below ground, for respiration, just as in 

 the case of animals. The parts below ground 

 require oxygen, and unless the soil is well aerated 

 the plant does not thrive, owing, in general, to the 

 soil being water-logged, or too close. Light is 

 necessary for carbon synthesis, and especially in 

 this connection are the red rays required; hence 

 the lack of active growth in winter under foggy or 

 smoky atmospheric conditions or a continuous 

 rainfall. 



Temperature and plant growth are intimately con- 



