NUTRITION 97 



natural conditions may at all times supply themselves with 

 phosphorus from its organic compounds in humus, in dead 

 bodies of plants and animals, and in living bodies. The 

 phosphates are less freely soluble in water than the salts of 

 the other necessary elements ; but since plants demand only 

 small quantities, and since the carbon-dioxide and possibly 

 also other acid secretions of their roots dissolve them more 

 rapidly than does pure water, plants secure under ordinary 

 conditions in nature all the phosphates needed. Manuring 

 with phosphates has to be resorted to only wiien the soil 

 has become impoverished by the removal of the crops culti- 

 vated upon it. 



In the plant phosphorus occurs not only as a constituent 

 element of living protoplasm, but also in those already 

 highly elaborated substances to be used in the construction 

 of protoplasm, and in the simpler compounds formed by the 

 breaking up of living or lifeless protoplasmic matter. The 

 compounds of phosphorus occur, therefore, in the plant as 

 solids as parts of its structure and as stored material; 

 also in solution as constructive material and as the pro- 

 ducts of destructive metabolism. 



SULPHUR, also a constituent element of protoplasm and 

 therefore always present in the ash of plants, is found in 

 even smaller proportions in the plant-body than phos- 

 phorus, as the following figures show * 



in lupine seeds 0.36% 



" potato leaves 0.43% 



tubers 0.24% 



" wood of trees 0.025% 



The source of sulphur for most plants is the various salts 

 of sulphuric acid commonly found in the soil and dissolved 

 in ordinary waters. Dependent plants the so-called para- 

 sites and saprophytes may perhaps obtain some sulphur in 

 the form of organic compounds. The moulds can use salts 

 of sulphurous acid, if present in sufficiently dilute solution, f 

 although they are nearly as poisonous to all higher plants 



* Frank, A. B. Lehrbuch, I., p. 586. 



f Quoted by Pfeffer from Nageli, Bot. Mittheilungen, Bd. 3, 1881. 



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