8 



TEXTBOOK OF PLANT PHYSIOLOGY 



This method consists in growing plants in an artificial medium, 

 the constituents of which are exactly determined, instead of in 

 natural soil whose composition is very indefinite and difficult to 

 ascertain. Pure quartz sand, small beads, or even platinum 

 chips may serve as such a medium, but water cultures are the 

 simplest and most convenient. In this case, the roots of the plant 



are not in soil but in water, in which 

 are dissolved all of the elements that 

 the plant is not able to obtain from 

 the atmosphere but which are neces- 

 sary for its development. 



Experiments first conducted in the 

 middle of the nineteenth century and 

 repeated many times afterwards, have 

 shown definitely that for the normal 

 development of the plant only a 

 limited number of elements must be 

 introduced into the solution. Thus, 

 one of the solutions most commonly 

 used for water cultures, Knop's solu- 

 tion, shows the following composition : 

 Ca(N03)2, four parts; KH2PO4, one 

 part; MgSO.4, one part; KC1, half a 

 part; and a small quantity of FeP04j 

 in 4,000 parts of water. Recently, 

 the American investigators, Shive and 

 Tottingham, have used a still simpler 

 solution consisting of three salts: 

 KH2PO4, Ca(N0 3 ) 2 , and MgS0 4 . The 

 importance of the separate elements 

 composing the nutrient solutions will 

 be discussed later. In this place we wish only to mention that 

 with due care the plant thrives just as well in such a solution 

 as in the most fertile soil and produces excellent yields (Fig. 1). 

 If from the number of elements composing the nutritive salts 

 one is excluded, for instance, K, N, or Mg, the plant will not 

 develop, or, more exactly, it will develop only as much as the 

 reserves stored in the seeds will allow. Wherefrom it may be 

 concluded, that all these elements normally must be absorbed 

 from the soil. But carbon is not among these elements, though, 



Fig. 



1. — A water culture (after 

 Nathansohn) . 



