ABSORPTION OF ASH-COXSTITUENTS 



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 (H3PO4), 

 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, the lack of which prevents chlorophyll formation; 

 they become pale and chlorotic, 3 even in the light, when grown without this 

 element. 



§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. I. 27 p. 

 Washington, 1901. 



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

 387-389- 1900-1002. 



« Molisch, 1892. [See note b, p. 51.] 



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

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



