194 VEGETABLE PHYSIOLOGY 



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 carbohy- 

 drates, 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 



