THE FOOD OF PLANTS. 13! 



Young leaves . Mature leaves 



Potash (dissolved 75-2 37-3 



| undissolved 24-8 627 



Lime [dissolved 69-5 72-4 



| undissolved 30*5 27*6 



Magnesia I dissolved ' 6 7 8 '3 



(undissolved 56-4 217 



Silica j dissolved 26-8 ifri 



| undissolved 73-2 83*9 



Phosphorus f dissolved 20*9 19*9 



pentoxide \ undissolved 79*1 8o'i 



Sulphur. It was mentioned above that sulphur is a con- 

 stituent of the proteid substances, and this is practically all 

 that is known as to its use in the economy. It is absorbed in 

 the form of sulphates, those of ammonium, potassium, magne- 

 sium, and calcium being the most advantageous. Sulphates 

 occur in solution in the cell-sap of organs in which metabolism 

 is actively proceeding and are doubtless formed in connexion 

 with the decomposition of proteid. 



Sulphur occurs in many products of metabolism, such as sulphurised 

 ethers, etc. These will be noticed hereafter. 



Phosphorus. Phosphorus is taken up by plants in the 

 form of phosphates. It enters into the composition of some 

 of the substances which constitute the organised parts of 

 plants, namely nuclein and plastin. 



According to Hoppe-Seyler, an organic substance containing phos- 

 phorus, Lecithin (C 44 H 90 NPO 9 ), occurs in actively growing cells of the 

 most various plants. This observation has not, however, been confirmed 

 by Naegeli in his researches upon the chemical composition of Yeast. 

 Lecithin has been found by Loew in the spores and by Reinke in the 

 plasmodium of ^Ethalium. 



Hoppc-Seyler has found that phosphorus is present in chlorophyll (see 

 infra, p. 155), forming, as he believes, an integral part of the molecule. 



Besides being a constituent of some of the substances 

 which enter into the organised structure of plants, phos- 

 phorus bears an important relation to certain of their meta- 

 bolic processes. Phosphates are always to be found in rela- 

 tion with living protoplasm, but the exact significance of this 



92 



