246 PLANT PHYSIOLOGY 



sulphates, including the slightly soluble calcium sulphate, 

 gjrpsum. The free sulphates enter the roots without undergoing 

 any change. They ascend with the water stream through the 

 vessels, reach the chlorophyll-bearing cells of the leaf, and here, 

 together with nitrogen and the carbohydrate products of photo- 

 synthesis, are used in the building of protein substances. At the 

 same time, reduction processes take place; for in the protein 

 molecule, sulphur is combined with carbon and hydrogen, not 

 with oxygen. The mechanism of the reduction of sulphur is 

 not understood as yet. 



When plant or animal tissues decay in the soil, sulphur is 

 spKt from the protein molecule in the form of hydrogen sulphide, 

 just as nitrogen is set free in the form of ammonia. But, 

 unhke ammonia, hydrogen sulphide is not available to plants, 

 being very toxic to the roots. It may become available, 

 however, when oxidized to sulphuric acid through the activity 

 of special bacteria, which have already been mentioned in Art. 



42. 



Like sulphur, phosphorus is available to plants only in the 

 highest form of oxidation as 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 in this respect may be regarded as substitution 

 products of phosphoric acids. Various esters of phosphoric acid 

 are found in plants. Some of these may represent the reserve 

 products of phosphorus, e.g., phytin, C6H6(OH2P03)6, which is 

 an ester of the cycHc hexatomic alcohol inositol 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 starvation. 

 Another group of phosphoric acid compounds is represented by 

 the phosphatides. Like the proteins, they form indispensable 

 compounds of the protoplasm. Moreover, they seem to play 

 an important role in determining the osmotic properties of the 

 cell, as has already been discussed in Art. 7. 



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 con- 

 centration of hydrogen ions in the plant (see Art. 62). Many 

 inorganic phosphates are found in those parts of the plant where 



