ABSORPTION OF MINERAL ELEMENTS BY PLANTS 245 



the cell, where they have a very vital function. Magnesium, for 

 instance, is found in the chlorophyll molecule. Hence, at least 

 some of the metals may be considered nutrient substances. 

 Phosphorus, as phosphoric acid, may be found in both inorganic 

 and organic form. In leaves, up to 50 per cent of the phosphorus 

 is found in the form of phosphates, and durjng the germination 

 of the seeds an almost complete disintegration of all organic 

 phosphorus compounds takes place. The separation of the ash 

 elements into two main groups as nutrient substances and as 

 regulators of vital processes, however, accords in general with 

 their role in the Hfe of the plant. It must not be forgotten that 

 the same element in some combinations may be considered as 

 belonging to one group; in other combinations, it may be in the 

 other group. 



56. Sulphur and Phosphorus, Their Metabolism in Plants 

 and Their Cycle in Nature. — Sulphur and phosphorus are just 

 as important to the plant as is nitrogen. Both of these elements 

 enter into the composition of protein substances, sulphur being 

 found in all proteins, phosphorus only in the more complex ones, 

 in the nucleoproteins. It is also present in the other complex 

 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 

 cytoplasm and nucleus, it is quite clear that sulphur and phos- 

 phorus are necessary for the plant. 



As has already been seen in examining respiration, compounds 

 of sulphur and phosphorus play a significant part also in the 

 performance of this most important process. In the beginning of 

 the oxidative disintegration of sugars, phosphoric acid combines 

 with them giving hexosephosphate, which is subjected to further 

 transformations w^ith the participation of respiratory enzymes, 

 while the compounds containing the SH group, such as cysteine 

 and glutathione, participate most actively in the oxidation- 

 reduction processes taking place during respiration. Many 

 authors explain the well-known highly toxic action of the cations 

 of heavy metals, such as mercury and silver, by their capacity of 

 replacing hydrogen in the sulphydryl group and thus excluding 

 it from the cycle of processes indispensable for respiration. 



Of the numerous compounds of sulphur, higher plants absorb 

 only the sulphate ion, the most important source being various 



