46 SOIL CONDITIONS AND PLANT GROWTH 



the soil conditions rather more favourable for any particular 

 crop. 



Some interesting results are obtained in glass house practice. 

 Tomato growers have learned to regulate water supply and 

 temperature in such a way as to produce compact bushy 

 plants, which they know by experience give more fruit than 

 the softer, larger plants, obtainable under other conditions. 

 Until the blossom is fertilised or has " set," therefore, vigorous 

 growth is not encouraged, and, in many cases, while the atmos- 

 phere is artificially damped, water is actually withheld from the 

 roots until, in the picturesque language of the grower, " the 

 plants cry for it ". After " setting," water is liberally supplied 

 and top dressings of manure are given. 



It is impossible at present to make a complete analysis of 

 these phenomena. The osmotic pressure of the plant cell is 

 known to alter with changes in moisture content of the soil, 

 and it may, and probably does, react on other characteristics 

 of the plant. Some of Iljin's results (141) are given in 

 Table VIII. 



Table VIII. — Change in Osmotic Pressure of Plant Cell with 

 Changes in Moisture Content of Soil. 



Ratio of Soil Moisture Osmotic Pressure in Normal Solution of NaCl.^ 



to Absolute Water Helianthus annuus. Zea Mays. 



Capacity of Soil. 



80 per cent. o*i4-o-i6 ciy 



60 „ o'25-o"28 0*19 



30 » 0-4I-0-45 0-49 



Effect of Soil Conditions on Consumption of Water by 



the Plant. 

 In the experimental work just described the water content 

 of the soil is kept artificially constant, thus eliminating the 

 effect of the rate of consumption by the plant. In natural 

 conditions, however, the supply is not maintained constant 

 and the rate of consumption therefore becomes an important 

 factor. Many determinations have been made of the weight 

 of the water transpired per gram of dry matter formed, the figure 

 being called ^ the transpiration coefficient. This mode of 



'o'l normal NaCl has an osmotic pressure of approximately 4 atmospheres. 



