240 PLANT PHYSIOLOGY 



present in a considerable concentration in plants. Bromine and 

 iodine, for example, accumulate in large quantities in the sea- 

 weeds. Some geologists, as for instance Vernadsky, attribute 

 to the accumulating power of living organisms a significant role 

 in the general circulation of the rare elements on the earth's 

 surface. 



The following elements are most often found in the ash of 

 plants: K, Na, Ca, Mg, Fe, Mn, Zn, B, Al, Si, P, S, and CI. 

 Their presence does not mean, however, that all of them are 

 really necessary. 



Analysis alone proves to be insufficient for solving the question 

 of which of these elements is really indispensable. The problem 

 requires the application of synthesis, and the basic synthetic 

 method employed for this purpose is the method of artificial 

 cultures. 



According to this method, instead of using natural soil, which 

 is of a very indefinite composition difficult to control, the plants 

 are grown in an artificial medium, all the constituent parts of 

 which are exactly determined. Well-washed pure quartz sand, 

 fine beads, and even scrap platinum may serve as such an 

 artificial substrate, but the most convenient is the method of water 

 cultures. In this case, the roots remain, not in soil, but in water 

 in which are dissolved all the elements that the plant cannot 

 obtain from the atmosphere but that nevertheless are absolutely 

 indispensable to its development. By the use of culture solu- 

 tions, it is possible to know the exact concentration of each ion 

 or substance. 



Experiments first carried out in the fifties of the last century, 

 and since then many times repeated, have shown quite definitely 

 that a very limited number of elements must be introduced into 

 the solution for the normal development of the plants. One 

 of the most commonly used solutions for water cultures, viz., 

 Knop's solution, contains the following salts: Ca(N03)2, 2 to 4 

 parts; KH2PO4, 1 part; MgS04, 1 part; KCl, K part; and a small 

 quantity of FeP04 dissolved in 4,000 parts of water. More 

 recently, Shive and Tottingham have used a solution composed 

 of only three salts, viz., KH2PO4, Ca(N03)2, and MgS04 with 

 the addition of traces of an iron salt. With sufficient care, 

 plants develop on such solutions as well as on the most fertile 

 soils and give excellent yields (Fig. 71). If but one of the ele- 



