ELEMENTARY CONSTITUENTS OF PLANTS 169 



follow that elements which are invariably present are therefore 

 absolutely necessary for the life of a plant. 



To determine with certainty which elements are indispensable 

 for proper nutrition and growth, cultivation experiments must be 

 carried out in soil or other media, the composition of which is 

 accurately known, and which can be regulated and kept under 

 control. This is best achieved by the methods of water-culture 

 and sand-culture, which consist in growing the plants in pure 

 water or in pure sand, to which are added compounds of the 

 various elements whose influence is to be studied. By means of 

 such experiments it has been proved that only ten elements are 

 really essential for the growtTi of green plants, namely, carbon, 

 hydrogen, oxygen, nitrogen, sulphur, phosphorus, potassium, 

 magnesium, calcium and iron : possibly to this list chlorine 

 should be added. 



All attempts to grow plants in soil, water or air from which any 

 one or more of these elements are excluded end in failure. The 

 other elements sometimes found in plant ash are superfluous; 

 even sodium and silicon, which are present in all plants growing 

 in ordinary soils, are not indispensable, for healthy specimens 

 capable of producing seed can be reared without them. 



Ex. 96. — Water-culture. — For the growth of plants in nutrient solutions 

 glass cylinders or wide-necked bottles holding about 600 or 700 c.c. of water 

 should be used. 



Before use the cylinder must be rinsed out first with nitric acid and then 

 thoroughly washed with distilled water. It should be fitted with a cork bung 

 through which two holes should be bored, one for the exit of the stem of the 

 plant to be grown, the other into which a short glass tube is fitted being con- 

 venient for adding water to replace that which is lost by transpiration. 



The solutions to be used must not contain more than from 2 to 5 grams of 

 dissolved salts in 1000 grams of water: a higher' concentration is detrimental 

 to growth. 



Moreover a slightly acid reaction should be maintained, alkaline solutions 

 being injurious. 



For complete nutrition the composition of the solution may vary consider- 

 ably so long as the essential elements are present in a suitable state for 

 absorption by the roots of the plants. The following solution contains all 



