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TEXTBOOK OF PLANT PHYSIOLOGY 



sary. Though a plant always contains a considerable amount of 

 silica in its ash, only an insignificant part of this is really necessary. 

 The remainder is evidently useless. Sometimes, however, the 

 accumulation of superfluous elements may play an accessory role. 



Thus, the silicic acid accu- 

 mulating in the cell walls 

 of cereals serves to stiffen 

 them. 



The common elements 

 used in nutrient solutions are 

 absolutely necessary for the 

 proper growth of plants. The 

 exclusion of any of them leads 

 to a check in development 

 and finally to death of the 

 organism. Figure 28 shows 

 clearly what conspicuous 

 differences may be observed 

 in the development of plants 

 in a complete nutrient solu- 

 tion and one from which a 

 single element is absent. It 

 seems to make little difference 

 which of the elements is ex- 

 cluded, all of them being 

 equally necessary. The slight 

 difference that may be ob- 

 served when various elements 

 are excluded depends not so 

 much upon their comparative 

 value as upon the amount of the particular element stored in 

 the seed and upon the quantity used for the development of the 

 plant. 



23. Importance of the Different Groups of Ash Constituents. 

 Nutrient Elements as Regulators of Vital Processes. — The con- 

 ception of the value of ash elements, which originated from the 

 results of the application of analytical methods, is from a physio- 

 logical point of view quite artificial. That certain ash constitu- 

 ents remain after the ignition of plant material does not necessarily 

 give an indication as to the part they may play in the living organ- 



Fig. 28. — Water cultures showing compara- 

 tive growth of plants: normal (in the center), 

 with one of the indispensable elements 

 absent (on the left and right) (after Pfeffer). 



