THE WATER-RELATION 



in 



to the roots, and the effect on growth studied. The usual arrangement 

 is shown in Fig. 75. A solution containing the above seven elements 

 in suitable form usually results in satisfactory growth. Knop's 

 culture solution is often employed in these experiments, and is made 

 up as follows : calcium nitrate, 1-0 gm. ; potassium nitrate, potassium 

 di-hydrogen phosphate and magnesium sulphate, 02 gm. each ; 

 ferric chloride, a trace ; distilled water, I litre. By suitable adjust- 

 ments of this formula, modified culture solutions can be prepared 



Fig. 75- 

 Water Cultures of Barley. The plants in the left-hand jar had been grown in a 

 solution containing all the essential elements. Certain elements were omitted from 

 the other jars, as labelled. The glass tubes allow of aeration of the solutions. 

 Photo. G. B. ( x i.) 



from which particular elements have been omitted. Thus instead 

 of magnesium sulphate, potassium sulphate could be added, giving 

 a magnesium-free solution. The growth of a plant in such a solution 

 could then be studied, and by this procedure it is found that omission 

 of any of the seven elements mentioned prevents proper growth (see 

 Fig. 75). In addition to these elements it now appears that certain 

 others, for example boron and manganese, must be accessible, to 

 some plants at least, in order to secure proper growth. These elements 

 are only needed in minute quantity, and it is probable that they are 

 present in sufficient amount as impurities in ordinary water cultures. 



