THE REQUIREMENTS OF PLANTS 

 Table IV. — Yield of Oats with Different Dressings of Phosphates. 



MiTSCHERLICH (202). 



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this method of treatment since it gives a constant independent of 

 the yield and having a definite mathematical meaning. 



The various factors we shall have to study are : (i) oxygen supply, 

 (2) light, (3) temperature, (4) water supply, (5) food supply, (6) harm- 

 ful factors. We must also distinguish between their effect on the three 

 stages of the plant's life, germination, active growth and maturation. 



The first five factors must all be present, or growth will not go on ; 

 an insufficiency of any one will operate as a limiting factor and put an 

 end to increased growth. Any injurious substance will act as shown 

 in Fig. 2 (p. 23). 



Oxygen. — The supply of oxygen to the leaves and stem is always 

 sufficient under agricultural conditions, but the supply to the root and 

 especially to the seed may often be inadequate. This is commonly 

 brought about by the presence of too much water in the soil, by too 

 compact a condition of the soil, or by excess of clay, whereby the per- 

 viousness is diminished. On the other hand, if the soil is too loose 

 plants fail to get a proper root hold or a proper water supply. 



Light. — H. T. Brown and Escombe (60) have shown that ordinary 

 daylight is more than adequate for the purpose of assimilation, and can 

 be reduced to one-twelfth without any ill-effect. It thus appears that 

 the plant is adapted to the worst light conditions it is likely to find. 

 Whether, however, growth would be as good in this diminished illumi- 

 nation has not been shown ; the experience of nurserymen indicates 

 that it is not. Only those rays (chiefly red) absorbed by chloro- 

 phyll are effective. The light penetrating the smoky atmosphere of 

 towns appears to have lost much of its activity, whilst light that has 

 passed through a green leaf is practically useless for vegetation. 

 Thus one crop will not grow in the shade of another : a dense crop 

 such as oats, wheat or maize shuts off the supply of light for 

 smaller weeds, and effectually prevents their growth, "smothering 



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