70 TEXTBOOK OF PLANT PHYSIOLOGY 



the nucleoproteins. It is also present in the other proteins of the 

 protoplasm that have been studied but little and often are known 

 by the general name of "plastins." As the nucleoproteins and 

 the plastins form vital parts of the living cell, it is quite clear that 

 sulphur and phosphorus are necessary for the plant. 



Of the numerous compounds of sulphur, plants absorb only the 

 sulphate ion, the most important source being various sulphates, 

 including the slightly soluble calcium sulphate (gypsum). 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 nitro- 

 gen and the carbohydrates are used in the building of protein sub- 

 stances. 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 split 

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

 as nitrogen is freed in the form of ammonia. But, unlike 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. 



These bacteria represent a special physiological group of 

 autotrophic chemosynthetic organisms of which we have spoken 

 already in Art. 8. Their physiology has been investigated by 

 Winogradsky. Through the oxidation of hydrogen sulphide they 

 secure energy, which is necessary for the decomposition of carbon 

 dioxide, and for the building of their own substance. This oxida- 

 tion is accomplished in two phases: first, hydrogen is oxidized to 

 water, the free sulphur remaining in the form of bright drops in 

 the cells of the bacteria. Then this sulphur is further oxidized 

 to sulphuric acid. 



Sulphur bacteria are of very great importance in nature. They 

 are present not only in the soil, but also in the upper layers of 

 water in ponds, ditches, and other basins, at the bottom of which 

 organic substances decay. These bacteria utilize the hydrogen 

 sulphide rising to the top. One may obtain an idea of their 

 activity from the fact that at the lower depths of such large bodies 

 of water as the Black and Azov seas the water is poisoned by 

 hydrogen sulphide, and no animal life is possible there. The 



