202 GROWTH 



A number of observations on the effect of X-rays on the 

 growth of plants have been made,* but since these radiations 

 do not appear to be a normal factor in the plant's habitat, 

 unless potassium is beneficial in its mild radio-activity,f 

 further mention is unnecessary in this reconnaisance. 



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 

 reduces transpiration ; adequate soil water is necessary if 

 transpiration be high ; the absorption of water and its con- 

 veyance to the transpiring surfaces is in part a question of 

 osmosis and this is bound up with permeability. In illustra- 

 tion some observations by Pearsall and Hanby % may be 

 mentioned. They found that increased growth took place 

 when the shoots of a variety of plants were grown in a culture 

 solution under increased pressure, 15 to 25 cm. of mercury; 

 contrariwise, small negative pressures are sufficient to reduce 

 or even stop growth. The available evidence indicates that 

 the positive pressure, which obviously leads to a greater tur- 

 gidity, promotes the rate of cell division and the formation of 

 a larger number of veins. Incidentally it may be mentioned, 

 for causal morphology does not form part of the present con- 

 sideration, that after treatment with negative or reduced 

 pressures, palmate leaves become smaller, more dissected, and 

 have reduced basal lobes when compared with similar leaves 

 after treatment with a positive pressure. 



Although biophysical problems are outside the present 

 province, it may be remarked that any factors which inter- 



* See Johnson : " Bot. Gaz.," 1926, 82, 373 ; " Amer. Journ. Bot. # " 

 1928, 15, 65. Van Heijningen, Blaauw and Hartsema : " Kon. Akad. 

 Wetens. Amsterdam," 1928, 26, 1. 



t See Blackman : " Ann. Bot.," 1920, 34, 299. 



J Pearsall and Hanby : id., 1926, 40, 83. 



