460 Messrs. A. D. Hall and C. G. T. Morison. [Dec. 22, 



act by facilitating the migration and utilisation in the grain of the initially 

 small store of phosphoric acid derived from the soil. 



But such an interpretation of the function of silica is not borne out if the 

 whole amount of phosphoric acid removed by the crop from the soil on each 

 plot be considered, instead of the proportion of phosphoric acid in the ash. 

 It has already been shown that the use of sodium silicate on the no 

 phosphoric acid plots, 1 and 3, results in a considerable increase of crop, and 

 as the grain of this increased crop is somewhat richer and the straw only a 

 trifle poorer in phosphoric acid than the grain and straw from the non- 

 silicated portions of the plots, it follows that the whole crop manured with 

 silica contains a greater total amount of phosphoric acid derived from the 

 reserves of phosphoric acid in the soil. This extra phosphoric acid derived 

 from the soil is itself sufficient to explain the greater yield brought about by 

 the silicate without attributing to the silica within the plant any specific 

 action in economising the phosphoric acid there present. If the function of 

 the silica were to replace the phosphoric acid within the plant and enable it 

 to be moved off to the active tissues and used over and over again, the larger 

 crop due to manuring with silicates would not contain any greater amount of 

 phosphoric acid, but the general growth of the plant, e.g., the dry matter 

 produced and the nitrogen assimilated, would be increased. Hence the ratio 

 of the phosphoric acid to the dry matter and to the nitrogen would be 

 lowered in proportion to the increased growth, conditions which are not 

 realised in the cases under examination, where indeed the ratio of phosphoric 

 acid to nitrogen is generally slightly raised by the applications of silicate. 



The results on the other hand indicate that the silicate gives the plant 

 such a stimulus as enables it to develop more vigorously and obtain more 

 phosphoric acid from the soil, and that all the consequences observed follow 

 from the increase of phosphoric acid thus brought about. 



Wolff and Kreutzhage held that the function of the silica was to enable 

 the plant to make fuller use of whatever phosphoric acid it had obtained 

 from the soil, the Eothamsted results indicate that its action takes place 

 earlier, in stimulating the plant to draw more efficiently upon the vast but 

 dormant reserves of phosphoric acid in the soil. 



3. Effect of Silica on the Development of Barley in 1904. 

 In order to study the question more closely it was decided in 1904 to trace 

 the effect of phosphoric acid and silica upon the development of the barley 

 on these plots at regular intervals from the time of flowering onwards. As 

 the effect of phosphoric acid had been most evident in forwarding the 

 maturation of the crop, it was considered that the later period of the growth 



