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- FiH. 40- Fig. 41- 



Young frond of wo] ffia Young frond of wolffia Anjjtfier species of 



growing out of older one. separating troin older one. wolffia, the two fronds 



still connected. 



Chemical analysis shows that certain mineral substances are 

 common constituents of j)lants. Tjy growing plants in different 

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

 mine what ones are necessarv 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 Hmits. 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 0.5*' 



Calcium sulpnate ° ■ 5 " 



Magnesium sulphate o ■ ,^ " 



Calcium phosphate o ■ 3 " 



The call ium phosjihale 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 alg:t. 



56. Several different jilants are useful for experiments in water cul'.ures, 

 as jieas, corn, beans, buc!:wheat, etc. The seeds of these jilants niav be 

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



