28 



PHYSIOLOGY. 



will be convenient to prepare some water cultures to illustrate 

 this and which can also be used later in our study of nutrition 

 (Chapter IX). 



Fig. 39- 



Young frond of wolffia 

 growing out of older one. 



Fig. 40. 



Young frond of wolffia 

 separating trom older one. 



Fig. 41- 



Another species of 

 wolffia ; the two fronds 

 still connected. 



Chemical analysis shows that certain mineral substances are 

 common constituents of plants. By growing plants in different 

 solutions of these various substances it has been possible to deter- 

 mine what ones are necessary constituents of plant food. While 

 the proportion of the mineral elements which enter into the com- 

 position of plant food may vary considerably within certain 

 limits, the concentration of the solutions should not exceed cer- 

 tain limits. A very useful solution is one recommended by Sachs, 

 and is as follows: 



55 . Formula for water cultures : 



Water. 1000 cc. 



Potassium nitrate o . 5 gr. 



Sodium chloride o . 5 " 



Calcium sulphate o . 5 ' ' 



Magnesium sulphate o. 5 " 



Calcium phosphate 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. 



56. Several different plants are useful for experiments in water cultures, 

 as peas, corn, beans, buckwheat, etc. The seeds of these plants may be 

 germinated, after soaking them for several hours in warm water, by placing 



