202 STRUCTURAL BOTANY 



always be chosen. To this water we add small 

 quantities of the chemical compounds with which we 

 wish to experiment. Plants never take up their food 

 directly in the form of chemical elements ; the 

 elements which the plant absorbs are always already 

 combined to form compounds. It will be well to 

 choose such compounds as are known to be present in 

 a natural soil. 



We may begin with a food-solution of the following 

 composition : 



Water (H 2 0) .... 1000 cubic centimetres 



Potassium nitrate (KN0 3 ) 1'Ogram 



Sodium chloride (NaCl) = common salt . '5 gram 



Calcium sulphate (CaSOJ 

 Magnesium sulphate (MgS0 4 ) 

 Calcium phosphate (Ca 3 (P0 4 ) a ) 

 Iron chloride (Fe 2 Cl 6 ) 



'5 gram 

 *5 gram 

 5 gram 

 a trace 



It will be seen that the amount of the various salts 

 is very small in comparison with the bulk of water. 

 Plants in nature take up their food, in so far as it 

 is supplied to them by their roots, in very weak 

 solutions. 



A seed (say, of the Maize, or Bean, or Pea, or Buck- 

 wheat) is allowed to germinate in damp sawdust. The 

 seedling is washed in distilled water, and is then fixed 

 in a split cork fitted into a wide-mouthed bottle, with 

 its root dipping into the food-solution with which the 

 bottle is filled. We have spoken of a food solution, 

 but one of the salts, calcium phosphate, is very slightly 

 soluble in water, and most of it will remain as a powder 

 at the bottom of the bottle. 



