THE CONSTITUENTS OF THE ASH OF PLANTS 181 
the plant, or whether the silicates are decomposed at once 
and the silica deposited in the cell-walls in which it is 
prominent. As it is most readily taken up in combination 
with sodium, this is unlikely, the sodium being, as we have 
seen, of very little, if any, use. It has been said that oats 
mature less fully and completely in the absence of silica, 
so that in the case of that particular plant there is some 
evidence of its aiding in metabolism, though no suggestion 
has been made as to the way in which it exerts its 
influence. It is possible that it may be of value also by 
protecting the plant from the depredations of animals or 
from the attacks of fungi, as it is mainly accumulated in 
the epidermis. 
The other elements of this group include chlorine, 
bromine, and iodine. A little of the former is of universal 
occurrence, but it may be due to its being taken up in 
conjunction with potassium. Water-culture experiments 
show, however, that in many cases it cannot be omitted 
altogether without injury to the plant. It has been asso- 
ciated by some writers with the translocation of carbo- 
hydrates, particularly in the buckwheat, a view which is 
based upon the observation that in its absence the chloro- 
plasts become abnormally filled with granules of starch. 
Bromine and iodine are chiefly found in marine plants, 
but their function is unknown. 
Manganese is a constituent of many plants. Till quite 
recently nothing was known about its influence on meta- 
bolism, but it now appears probable it plays a part in 
various oxidative processes, which are carried out by a 
somewhat widely spread enzyme known as Laccase, whose 
normal function is, however, at present obscure. 
The elements of the last group are numerous; they 
vary with the composition of the soil in which the plants 
are growing, and appear to subserve no useful purpose. 
Many of them in even moderately dilute solutions are 
extremely poisonous, so that they must be absorbed in a 
high state of tenuity. Their presence shows that the 
