62 OF THE ASSIMILATION OF CARBON. 



lime, in the absence of matter containing nitrogen. 

 We may, indeed, conclude, that the salt of lime acts 

 as a poison, since the development of plants will ad- 

 vance further in pure water, when lime and carbonic 

 acid are not used. 



Moist flowers of sulphur attract oxygen from the 

 atmosphere, and become acid. Is it possible that a 

 plant can grow and flourish in presence of free sul- 

 phuric acid, with no other nourishment than carbonic 

 acid ? It is true, the quantity of sulphuric acid 

 formed thus in hours, or in days, may be small, but 

 the property of each particle of the sulphur to absorb 

 oxygen and retain it, is present every moment. 



When it is known that plants require moisture, 

 carbonic acid, and air, should we choose, as the soil 

 for experiments on their growth, sulphate of barytes, 

 which, from its nature and specific gravity, com- 

 pletely prevents the access of air. 



All these experiments are valueless for the deci- 

 sion of any question. It is absurd to take for them 

 any soil, at mere hazard, so long as we are ignorant 

 of the functions performed in plants by those inor- 

 ganic substances which are apparently foreign to them. 

 It is quite impossible to mature a plant of the fam- 

 ily of the Gramineaij or of the Equisetacece, the solid 

 framework of which contains silicate of potash, with- 

 out silicic acid and potash, or ar plant of the genus 

 Oxalis without potash, or saline plants such as the 

 saltworts (^Salsola and Salicornia) without chloride 

 of sodium, or at least some salt of similar proper- 

 ties. All seeds of the GraminecB contain phosphate 

 of magnesia; the solid parts of the roots of the 

 althcBa contain more phosphate of lime than woody 

 fibre. Are these substances merely accidentally 

 present ? A plant should not be chosen for experi- 

 ment, when the matter which it requires for its 

 assimilation is not w^ell known. 



What value, now, can be attached to experiments 

 in which all those matters which a plant requires in 

 the process of assimilation, besides its mere nutri- 



