IMPORTANCE OF FOOD 129 



The vegetative and reproductive phases of a plant are an- 

 tagonistic, for which reason conditions which promote the one 

 will degrade the other. Thus generally a low degree of 

 humidity favours flower production ; red light rays accelerate 

 the formation of reproductive shoots whilst blue rays retard, 

 but darkness does not inhibit the formation of flowers at low 

 temperatures, it is only at higher temperatures that intermit- 

 tent light and darkness can be made to effect the inhibition.* 



NUTRITION. The importance of food is so obvious a 

 factor governing growth that its consideration would appear 

 to be unnecessary : but the significance of certain raw materials, 

 more especially inorganic substances, is so marked as to 

 warrant some mention. 



The following table, results obtained at Rothamsted,f 

 generally illustrates the action of various chemical fertilizers 

 on field crops : 



WHEAT. (Average yield 1852-1912). 



Grain in Straw in 



Bushels. Cwts. 



Complete minerals 32*1 32-9 



Ammonium salts + superphosphate . . . 22*9 22-3 



Ammonium salts + superphosphate + sodium sul- 

 phate 29'! 28*0 



Ammonium salts + superphosphate + potassium 



sulphate 31*0 31-5 



Ammonium salts + superphosphate + magnesium 



sulphate 28-8 28 



GRASS. (Cut for hay every year). 



1918. 1919. 1920. 



Complete mineral manure .... 45^2 32-9 46*0 cwts. 



Minerals without potash .... 25-9 197 27-3 



Complete minerals + ammonium salts . 46*7 53-2 49-6 



Minerals without potash + ammonium salts 33-5 34-5 32*6 



In this connexion the relative abundance of colloids in the 

 soil would appear to be of importance ; thus Jennings J finds 

 that colloids added to the culture medium promoted or de- 

 pressed the growth of wheat seedlings according to the specific 

 colloid employed and the concentration of the culture medium. 

 Agar, for instance, in the presence of low concentrations of 



* See Klebs : " Flora," 1918, n, 128. 



t For these we are indebted to Dr. W. Brenchley. 



t Jennings: "Soil Science," 1919, 7, 201. 



VOL. II. 9 



