50 THE WORK OF THE ROOTS [chap. 



this view is erroneous ; there is evidence that the acid 

 sap can never reach anything outside the unbroken root, 

 and all the actions, including the etching of the marble, 

 are quite explained by the excretion of carbon dioxide, 

 which, we know, is always taking place from the root. 

 As a solution of carbon dioxide in water acts like a weak 

 acid and is much more effective than pure water in 

 dissolving mineral matter, the roots of the plant have a 

 distinct effect in rendering the materials in the soil 

 available as food for the plant ; this point will be 

 considered later when dealing with the soil. 



We may now proceed to make use of the method of 

 water cultures, to determine what are the mineral sub- 

 stances, etc., taken by the plant from the soil, and which 

 of them are essential to its growth. When dealing with 

 the composition of the plant, we have already seen that a 

 comparatively small range of elements is to be found 

 there, and that all plants are alike in containing besides 

 carbon, hydrogen, and oxygen, also nitrogen, sulphur, 

 phosphorus, chlorine, and often silica, with potash, 

 soda, magnesia, lime, and iron among bases. Only 

 these elements can be essential to the plant : and 

 whether all of them are necessary can be ascertained 

 by growing a set of plants in water cultures and 

 omitting from successive bottles each element in turn. 

 Taking, for example in bottle i the standard 

 solution which we have already employed ; in 2 the 

 potassium nitrate may be omitted, thus giving a 

 solution without nitrogen (the potassium will be still 

 furnished by the potassium phosphate) ; in 3 the 

 potassium phosphate is omitted, to get a solution con- 

 taining no phosphoric acid; in 4 the potassium 

 phosphate and nitrate are replaced by the corresponding 

 sodium salts ; and so on. The seedling barley or maize 

 plants are inserted in the usual way, with the result 



