22 GENERAL BOTANY 



are absolutely necessary to all plants, and that other 

 elements, even such as are chemically very similar, 

 will not do instead. 



We may quite easily grow almost any plant in water 

 by filling wide-mouthed jars with water containing 

 salts of these elements in very dilute solution, and 

 fixing young seedlings to corks, so that the roots are 

 always under water. Fresh solution must, of course, 

 be added to make up for what is used up and evaporated, 

 and it is best to keep the roots dark (by covering the 

 sides and tops of the jars with thick brown paper) and 

 to blow or force air through the water occasionally 

 to keep it healthy. Sometimes too the water gets acid 

 and to prevent that it is good to add a little slaked lime 

 (chunam) which will neutralize any excess of acid. 

 About one part in five thousand (of water) of such 

 salts as POTASSIUM NITRATE, MAGNESIUM SULPHATE 

 and CALCIUM PHOSPHATE with the merest trace of 

 FERRIC CHLORIDE will be sufficient. It will be no- 

 ticed that among these are two elements not mentioned 

 as occurring in the ash nitrogen and chlorine. Chlo- 

 rine, it has been found, is not necessary for plant 

 growth, though it appears to help keep the roots 

 healthy, but nitrogen is absolutely necessary, and is in 

 fact in ordinary farming processes the most important 

 of all the elements. It does not occur in well-burnt 

 ash simply because its oxides are volatile and when 

 its compounds are burnt, it passes away in the smoke 

 as nitrogen or its oxides. So we must always add 

 compound of nitrogen to those represented in the ash. 



These elements must, of course, enter the plant from 

 the soil there is no other way and they are taken 



