THE CONSTITUENTS OF THE ASH OF PLANTS 183 



The second group of ash constituents includes four 

 metals which are essential to all plants, viz. potassium, 

 magnesium, calcium, and iron. Probably these act only 

 indirectly in the constructive processes, though there is 

 some evidence that they may be integral constituents of 

 living substance. They do not enter into the composition 

 of proteins. 



Potassium is absorbed in a variety of compounds, of 

 which the nitrate and the chloride are the most advan- 

 tageous. The part which it plays is not at all well under- 

 stood. It may enter into the composition of protoplasm, 

 for it is especially abundant in embryonic tissues. It has 

 been thought to be connected with the construction of 

 carbohydrates, but in what way is not known. It occurs 

 in greatest quantity in the organs in which the formation 

 and storage of these bodies are most actively carried out, 

 viz. leaves, tubers, seeds, &c. 



Magnesium has a distribution much like that of po'as- 

 sium, and, as well as calcium, is thought by some botanists 

 to enter into the composition of protoplasm. It may be 

 absorbed in various combinations, but the chloride is the 

 least advantageous. Calcium is essential to all green 

 plants, but fungi do not always require it. Little of it 

 relatively is found in young tissues, but greater amounts 

 are present in adult ones. Its function is not understood, 

 but it is useful in neutralising oxalic acid. It is prominent 

 in the cell-wall, part of which even in the very young state 

 consists of calcic pectate. In older cells the middle lamella 

 appears to consist entirely of this substance until lignification 

 is complete. Calcium may be absorbed in the same com- 

 binations as magnesium. 



As has been already mentioned, the most evident function 

 of iron is to assist in the formation of chlorophyll. As it is 

 not contained in the pigment, its influence here can only be 

 indirect. It may be associated in some way with the proto- 

 plasmic basis of the plastid, so that the latter in its absence 

 is thrown into a pathological condition and ceases to form 



