472 



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



On August 5, although the plants were by no means fully mature, it was 

 necessary to harvest them because of an attack of aphis. When dried they 

 gave the following results : — 



Table V. — Barley in Water Cultures. Yield on August 5, 1904. 



Plot. 



Number of 



Number of 



Dry matter. 



ears. 



grains. 



Grain. 



Straw and roots. 



Total. 



1 



4 







grammes. 

 



grammes. 

 35 -17 



grammes. 

 35 -17 



2 



7 



5 







31 -87 



31 -87 



3 



18 



177 



5-26 



50-79 



56 -05 



4 



27 



272 



9 63 



56 26 



65 -89 



Assuming that on June 9, when the treatment was varied, all of the plants 

 were approximately equal, it will be seen that the extra phosphoric acid 

 added to Nos. 3 and 4 allowed them to double their weight during the 

 remaining period of growth. The silica alone added to No. 2 did little to 

 enable the plant to make better use of the restricted amount of phosphoric 

 acid already in the plant, for although the formation of ears seems to have 

 been a little forwarded, the few grains that were produced possessed no 

 sensible weight. When, however, silica is provided in the presence of phos- 

 phoric acid, No. 4 compared with No. 3, it brings about a considerable increase 

 of growth and an accelerated formation of grain — just such a change, in fact, 

 as would be brought about by an increased assimilation of phosphoric acid. 

 In fact, these cultures demonstrate that although silica cannot replace phos- 

 phoric acid, nor even economise and make more effective a restricted supply 

 already within the plant, it will stimulate the plant to assimilate a greater 

 amount of phosphoric acid should' that be obtainable from the medium in 

 which the plant is growing. Hence, when applied to a silica plant on a soil 

 impoverished in phosphoric acid, it has the same effect in increasing and 

 accelerating the formation of fruit as would result from a direct application 

 of phosphoric acid. 



It might be supposed that the action takes place within the soil itself, that 

 the sodium silicate in some way attacks the insoluble phosphates of the soil 

 so as to render them more available for the plant, much as an application of 

 lime or gypsum will liberate an increased supply of potash from the soil. On 

 chemical grounds it is difficult to see how such an action should occur, nor do 

 the results with water cultures bear out such a view. To obtain further 

 evidence on this point, samples of soil from the eight plots in question were 



