ABSORPTION OF ASH-CONSTITUENTS 



85 



Sulphur is a necessary element because it is essential to the formation of 

 proteins, which are so important in plants. It must be supplied as the sulphate 

 of one of the essential metals; all other compounds of sulphur are injurious. 

 It cannot be replaced by any other element. 



Phosphorus also is necessary. It is a constituent of nucleins (a special 

 group of proteins), and of phosphatides. It may be introduced in the solution 

 only as one of the phosphates 

 of the tribasic acid (H ; »P0 4 ), 

 since other phosphorus com- 

 pounds have been found to 

 be harmful. It cannot be re- 

 placed by any other element. 



Potassium is also abso- 

 lutely essential. It accom- 

 panies carbohydrates and is 

 supposed to promote their 

 formation. 



Calcium is likewise neces- 

 sary, especially for normal 

 leaf development. Some 

 plants without chlorophyll 

 (moulds) can exist without 

 calcium, 1 and non-green 

 phanerogams contain much 

 less calcium than do green 

 plants. 2 



Magnesium is also neces- 

 sary; it accompanies pro- 

 teins and is contained in 

 chlorophyll. 



Finally, plants need iron, for chlorophyll formation does not occur without 

 an adequate supply of this element; they become pale and chlorotic, 3 even in 

 the light, when grown without iron. 



§3. Importance of the Non-essential Ash-constituents. — Plant ash contains 

 appreciable quantities of other elements than the absolutely essential ones, and 

 these are not to be considered as entirely without physiological effects. Each 

 ash-constituent must be considered as exerting some slight effect in the plant, 

 either injurious or beneficial. If plants develop apparently normally in a nutri- 

 ent solution without a given element, it does not necessarily follow that this 

 element, if present, might not exert some beneficial influence. 



Silicon, for example, is abundant in many plants. Nevertheless, experi- 

 ments with various plants in artificial media have shown that even the grasses 



1 Loew, Oscar, Liming of soils from a physiological standpoint. U. S. Dept. Agric. Bull. 1. 27 p. 

 Washington, 1901. 



2 Aso, K., On the lime content of phanerogamic parasites. Bull. Coll. Agric. Imp. Univ. Tokyo 4: 

 387-389. 1900-1902. 



3 Molisch, 1892. [See note b, p. 52.] 



Fig. 51. — Portion of a cross-section through a rye stalk. 

 At left, lodged; at right, normal. (After Koch.) 



