178 VEGETABLE PHYSIOLOGY 
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 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. 3 
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 plastid, 
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. 
