THE CONSTITUENTS OF THE ASH OF PLANTS 191 



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

 understood. 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 it affects it 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 potas- 

 sium, and, as well as calcium, is thought by some bota- 

 nists 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 promi- 

 nent in cell-walls, part of which even in the very young 

 state consist 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 combinations as magnesium. 



As has been already mentioned, the most evi- 

 dent 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 protoplasmic basis of the plastic], 

 so that the latter in its absence is thrown into a patho- 

 logical condition and ceases to form the colouring matter. 

 The influence both of the metal and of light in this 

 particular may consequently be similar. That it is asso- 

 ciated with the plastid does not appear improbable in 

 view of some observations of Macallum's that iron is 

 always found in direct relationship with the chromatin of 

 the nucleus, of which it appears to be an integral part. 

 There is here evidence of a close association between the 

 metal and some forms of living substance. 



