INTRODUCTORY 7 



quantities, its effects are highly deleterious to the majority 

 of plants. Sodium again is entirely indifferent. Such 

 substances are regarded as accidental because they are 

 probably taken up by the plants, to a large extent, in 

 combination with the other elements, e.g., potassium, 

 calcium, etc., which have an active role in the vital pro- 

 cesses of the organism. 



Sources of the Constituents. Whatever is found in the 

 plant must have been derived at least proximately either 

 from the soil or from the air. Sulphur, hydrogen, and the 

 elements of the ash are not constituents of the air, and 

 must therefore be obtained from the soil. The source of 

 the carbon, nitrogen, and oxygen, which occur normally 

 both in the air and in the soil, can be determined only by 

 experiment. A simple plan, generally applicable to pro- 

 blems of this description, is to grow the plants in an arti- 

 ficial soil, or medium of some kind to which they are 

 entirely indifferent. The effects of various substances can 

 then be estimated by introducing them to, or withholding 

 them from the medium. 



Sand Cultures. Many different substances have been 

 employed as mediums, but, for most purposes, pure 

 sand (powdered quartz) is the most suitable. It is cheap, 

 easily procured and purified. It is one of the largest con- 

 stituents of most ordinary soils, and the plants are, there- 

 fore, to that extent, grown under normal conditions. 

 If the sand be kept moist and at a suitable temperature; 

 seeds sown in it readily germinate and continue to grow 

 until the nourishment stored up in them is exhausted. 

 Further development cannot take place, and the plants die 

 for lack of food, for, it is assumed, there is none in the 

 soil. If the experiment be varied by mixing with the sand 

 small quantities of the various constituents of plants, 

 the seedlings will continue to thrive and grow into 

 fully developed plants which may, in time, bear seed. 



