THE ABSORPTION OF MINERAL ELEMENTS 71 



upper part of the water, however, does not contain even traces of 

 this gas, due to the activity of vast numbers of sulphur bacteria, 

 which are distributed in an unbroken layer to the depth of 200 m. 



Similarly to sulphur, phosphorus is available to plants only in 

 the highest form of oxidation, namely, salts of orthophosphoric 

 acid. It does not undergo quite such complex transformations in 

 the plant body, however, for it is found in the same oxidized state 

 in the proteins, which may be regarded as substitution products of 

 phosphoric acids. Besides, in plants various esters of phosphoric 

 acid are found. Some of these may represent the reserve prod- 

 ucts of phosphorus, as for instance pbytin, C 6 Ho(OH2P03)6, which 

 is an ester of the cyclic hexatomic alcohol, inosite, C 6 Ho(OH)6, 

 and contains 6 molecules of phosphoric acid. Like many other 

 plant reserve products, phytin is digestible by animals. Hence it is 

 widely used in the diet as a remedy in cases of phosphorus starva- 

 tion. Another group of phosphoric acid compounds is represented 

 by the so-called "lipoids," fatlike substances containing phosphoric 

 acid as well as some basic nitrogen. The lecithins and phospha- 

 tides also belong to the group of lipoids. Like the proteins, they 

 form indispensable compounds of the protoplasm. Moreover, they 

 seem to play an important role in determining the osmotic proper- 

 ties of the cell, which we shall discuss in the next chapter. 



A considerable part of the phosphoric acid, sometimes 50 per 

 cent of the whole amount, remains in the plant in ionic form, and 

 plays a very important role in the regulation of the active concen- 

 tration of hydrogen ions in the plant (Art. 36). Many inorganic 

 phosphates are found in those parts of the plant where an energetic 

 mobilization of the reserve substances takes place, for instance, 

 in germinating seeds. 



When plants or animals decay, the phosphoric acid is liberated 

 largely in the form of inorganic salts, which are available to plants. 

 Complex organic substances of phosphorus cannot be utilized by 

 plants without decomposition. This has been proved by numerous 

 sterile cultures of plants which included such substances. If the 

 cultures were successful, that is, if they remained sterile to the end, 

 the plants developed very poorly. If by accident they were 

 infected, which caused a breaking down of the complex phosphorus 

 compounds, then the plants grew much better. 



25. The General Role of Cations and Their Specific Activity. — 

 The role of the cations in the life of the plant is more complex 



