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 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 



