WORK OF THE ROOT 



55 



suitable food for succeeding generations. It is important, 

 however, that a suitable balance should be maintained 

 between the organisms in the soil, if higher plants are to 

 thrive in it. From what we have seen, the food which an 

 ordinary green plant can take up from the soil must be 

 a weak solution of inorganic substances, e. g. compounds 

 which form the mineral matter of the soil as distinct from 

 organic substances which are carbon compounds built up 

 by living organisms, e. g. cellulose, sugar, starch, and 

 proteins. 



Water -cultures. If soil-water or ordinary tap-water be 

 placed in a shallow vessel and covered by a sheet of paper 

 so as to prevent access of dust or other matter, and allowed 

 to evaporate, a sediment will be left at the bottom of the 

 vessel, consisting of substances which were held in solution. 

 These, when analysed, are found to consist of a number of 

 mineral salts which, in suitable proportions, are able to 

 sustain a green plant grown under the usual conditions 

 of air and light. Such a sediment, however, may possibly 

 contain substances not necessary to the plant. 



Experiments have been made to determine which of 

 these compounds are essential. The following solution con- 

 tains the inorganic substances commonly present in a 

 natural soil : 



Distilled water (HO) 



Potassium nitrate (KNOJ 

 Sodium chloride (NaCl) 

 Calcium sulphate (CaS0 4 ) 

 Magnesium sulphate (MgSOJ 

 Calcium phosphate (Ca.,(P0 4 ) ) 

 Ferric chloride (FeCl 3 ) 



Such a solution is known as a normal water-culture solu- 

 tion, and in it plants may be grown up to the flowering and 

 fruiting stages (Fig. 25, b). 



