WATER-ABSORPTION AND TRANSPIRATION. 



109 



142. Essential Elements of Plant-Pood. All the 



elements named in Art. 137, and several others, occur in the 

 composition of plants and their food. 

 In fact, nearly all the chemical elements 

 known to occur in soil, water, and air 

 have been detected in analyses of 

 plants. It has been found by experi- 

 ment, however, that for most green 

 plants the essential elements i.e. 

 the elements absolutely necessary for 

 healthy growth are carbon, oxygen, 

 hydrogen, nitrogen, sulphur, phos- 

 phorus, calcium, potassium, mag- 

 nesium, and iron. 



143. Water Culture. It has long 

 been known that a plant can be grown 

 in water containing salts in which all 

 these elements are present with the 

 exception of carbon. That carbon is 

 essential is easily proved, as we have 



seen, yet this element need not be Fig. ss. A riant of Buck- 

 present in the solution supplied to the ^ ^ ow a n gjjjjj 

 roots. That all the other elements are solution, 

 essential, and must be supplied to the 



roots, has been shown by the method of water culture. 

 Several plants of the same species are grown in glass jars 

 (Fig. 38), their roots dipping into a culture solution. With 

 most plants there is healthy growth only if the solution 

 contains potash, lime, and magnesia, combined with nitric, 

 phosphoric, and sulphuric acids, together with a trace of an 

 iron salt. 



* 144. Experiments with a complete Culture Solution. 



Get some large glass jars, each holding at least a quart, for water- 

 culture experiments. Sachs' solution consists of 2 grammes of 

 potassium nitrate, 1 gramme each of sodium chloride, calcium sulphate, 

 magnesium sulphate, and calcium phosphate, and a drop or two of 

 iron chloride (or iron phosphate) to 2 litres of distilled water. Knop's 

 solution, which is perhaps better, consists of 2 grammes of calcium 

 nitrate and 0'5 gramme each of potassium nitrate, magnesium sul- 

 phate, and potassium phosphate, with iron as before, in 4 or 5 litres 

 of water. Perhaps the best plan is to get 4 oz. of calcium nitrate and 



