CHAPTER VII. 



HOW PLANTS OBTAIN THEIR LIQUID FOOD. 



I. WATER CULTURES. 



67. How constituents of plant food are determined. We 



are now ready to inquire how plants obtain food from the soil 

 or water. Chemical analysis shows that certain mineral sub- 

 stances are common constituents of plants. By growing plants 

 in different solutions of these various substances it has been 

 possible to determine what ones are necessary constituents of 

 plant food. While the proportion of the mineral elements 

 which enter into the composition of plant food may vary con- 

 siderably within certain limits, the concentration of the solutions 

 should not exceed certain limits. A very useful solution is one 

 recommended by Sachs, and is as follows : 



68. Formula for solution of nutrient materials. The pro- 

 portions of the ingredients are here given. A larger quantity 

 than IQOOCC may be needed. 



Water 1000 cc. 



Potassium nitrate 0.5 gr. 



Sodium chloride 0.5 " 



Calcium sulphate 0.5 " 



Magnesium sulphate 0.5 u 



Calcium phosphate 0.5 " 



The calcium phosphate is only partly soluble. The solution which is not 

 in use should be kept in a dark cool place to prevent the growth of minute 

 algae. 



Demonstration 13. 



69. To prepare the seedlings in water cultures. Several different plants 

 are useful for experiments in water cultures ; peas, corn, or beans are very 



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