THE REQUIREMENTS OF PLANTS 



47 



TABLE XXI. EFFECT OF SILICATES ON THE GROWTH OF BARLEY, 1864-1904. 



ROTHAMSTED. 



Morison (119) conclude that silicates act by causing an increased as- 

 similation of phosphoric acid by the plant, the seat of action being in 

 the plant and not in the soil. 



Absence of Injurious Substances. We have seen that many 

 salts have a toxic effect if given alone to the plant, but for our pur- 

 pose we need consider only those causing injury in presence of other 

 compounds. Two cases arise in practice : some substances are in- 

 jurious in small quantities, others only in excess. 



Substances Injurious in Small Quantities : Adds. Cultivated plants 

 will usually not grow in too acid or too alkaline a medium, but prefer 

 something more nearly neutral. In water cultures it is necessary to 

 begin with a faintly acid solution because of the formation, as growth 

 proceeds, of sodium and potassium carbonates (see p. 136) : in soils, how- 

 ever, certain changes set in that not only obviate the need for acidity 

 but necessitate the presence of calcium carbonate. 



The unsuitability of the atmosphere of industrial towns has been 

 traced in part to the presence of acids, which affect the leaves as well 

 as the roots. Wieler J found that assimilation of carbon dioxide was 

 profoundly modified by sulphur dioxide, most injury being done in 

 moist weather when the stomata were more widely opened and the 

 gas could readily enter the leaf tissues. Crowther and Ruston (72) 

 obtained the following yields from pots of Timothy, showing that acid 

 water gradually kills the plant : 



1 Bled. Zentr., 1908, xxxvii., 572, 



