186 VEGETABLE PHYSIOLOGY 
consequently to the maintenance of their vertical position. 
This is, however, not the case; their rigidity is dependent 
on the degree of development of their harder tissues, and 
the absence of silica makes but little difference to them. 
Silicates, when added in quantity to the soil in which green 
crops are growing, have no marked effect upon the amount 
of silica which is subsequently present in the straw. It is 
uncertain whether the silica enters into the metabolism of 
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 possihle that it may be of value also by protecting 
the plant from the depredations of animals or from the 
attacks of fungi, as it ig 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 jwhich are carried out by a 
