THE CONSTITUENTS OF THE ASH OF PLANTS 188 
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 ig 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 ig 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 lame la 
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 
1s thrown into a pathological condition and ceases to form 
