INFLUENCE OF WATER 203 



rupt the normal osmotic adjustments of a plant must adversely 

 affect its vegetative growth. Thus Dufrenoy has pointed out * 

 that growth is a function of at least six factors and a variation 

 in any one of these will influence the velocity of growth. He 

 considers that growth rate depends upon the balance between 

 the sum of imbibition, the osmotic pressure of colloids and of 

 the salts of the cell sap, and the sum of the tension of mem- 

 branes, the osmotic pressure of the salts of the surrounding 

 medium and the mechanical resistance of the medium. 



It is obvious that water content and sap concentration are 

 interlocked factors. Reed f investigated the growth of shoots 

 of the apricot, orange and walnut throughout the growing 

 season and at the same time followed the concentration of the 

 expressed sap. In general terms, he found that rapid growth 

 was associated with a low concentration of sap, higher con- 

 centrations are associated with the formation of fruit buds as 

 well as with slow vegetative growth. In fact it is a matter of 

 almost common observation that the vegetative and repro- 

 ductive phases of a plant are antagonistic, for which reason 

 conditions which promote the one will degrade the other.J 



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 importance of various salts, especially phosphate and 

 nitrate in combination with such essential bases as potassium, 

 calcium and magnesium, on the growth of plants is a common- 

 place of the physiological laboratory and is easily demon- 

 strated by the use of sand and water cultures in controlled 

 conditions. By using different combinations of salts in such 

 experiments, the effect of single nutrient ions on plant yield 

 can be ascertained, a subject of some complexity. § 



* Dufrenoy : " Rev. gen. Sci.," ioi8, 29, 323. 

 f Reed : " Journ. Agric. Res.," 1921, 21, 81. 

 + See Klebs : " Flora," 1918, II, 128. 



§ See Johnston : " Univ. Maryland Agric. Exp. Sta.," 1924- Gregory: 

 " Proc. Roy. Soc," B, 1928, 102, 311. 



