128 GROWTH 



In general terms, the more the short rays, especially the ultra- 

 violet, were removed, the greater the growth up to a certain 

 limit It also was observed that earlier flowering was pro- 

 moted by culture under 3 and 4 of the series. 



With regard to energy other than light, field experiments 

 with wheat and other crops show the beneficial action of 

 overhead electric discharge.* In the year 1915 the increase 

 in grain and in straw was 30 and 50 per cent respectively 

 greater than the control, and in 1916 the corresponding figures 

 were 49 and 88 per cent. The effect of the discharge also 

 showed itself in the year subsequent to its application ; thus 

 in the clover and grass crop there was a marked increase in 

 1916, the year following the treatment. The reasons for the 

 increase following the stimulation have not yet been found. 



WATER. Common experience shows that growth is only 

 possible provided that the living cells are in a turgid condition, 

 wherefore circumstances which promote this condition will 

 promote vegetative growth, but not necessarily reproduction. 

 The circumstances to which allusion is made are humidity, 

 adequate soil water, absorption of water and transpiration. A 

 humid atmosphere promotes vegetative growth in that it re- 

 duces transpiration ; adequate soil water is necessary if trans- 

 piration be high ; the absorption of water and its conveyance 

 to the transpiring surfaces is in part a question of osmosis and 

 this is bound up with permeability. Although biophysical 

 problems are outside the present province, it may be mentioned 

 that any factors which interrupt the normal osmotic adjust- 

 ments of a plant must adversely affect its vegetative growth. 

 Thus Dufrnoy has pointed out f 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 membranes, the osmotic pres- 

 sure of the salts of the surrounding medium and the mechani- 

 cal resistance of the medium. 



* Blackman and Jorgensen : " Journ. Board Agric.," 1916, 23, 671 ; 1917, 



24,45. 



t Dufr<noy : " Rev. gn. Sci.," 1918, 29, 323. 



