The Various Substances Made by Plants 135 



them up in out of the way places, though they may ultimately 

 be partially removed by the fall of the leaves and the bark. 



There remains one other important phase of our subject. It 

 concerns the indispensability of certain elements for healthy 

 metabohsm, although they do not enter into the composition 

 of any of the substance manufactured. Everybody knows that 



Lacks, — all potas- cal- nitro- phos- mag- iron nothing 

 sium cium gen phorus nesium 



Fig. 43. — Illustration of the method and results of water culture. The plants are Corn, 

 all started at the same time. (Copied from a wall-chart by Errera and Laurent.) 



potash (potassium) is thus indispensable, to such a degree that 

 it must often be added as a fertilizer to soils; but its symbol (K) 

 is not found in any of the formulae cited in this chapter. The same 

 is true of the elements calcium, magnesium and iron, and probably 

 sodium and chlorine, all of which are indispensable to the healthy 

 growth of most or all plants, but none of which enter into the 

 composition of the most miportant plant substances. Naturally a 



