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THE BIOLOGY 

 OF PLANT GROWTH 



James Bonner 



CALIFORNIA INSTITUTE OF TECHNOLOGY 



This discussion will concern plant biology and what we have come to 

 understand about plant growth and development. I wish too to assess 

 some of the problems which it seems to me confront us still. I will 

 discuss these matters in terms of a question: What are the things that 

 determine the ultimate limits of plant productivity on earth? This is 

 an interesting question. The fact of plant growth is of course central to 

 life on earth as we know life today. All flesh is grass, and all atmos- 

 pheric oxygen is the breath of chloroplasts. The rate at which plants 

 conduct their affairs therefore sets an upper limit on the amount of 

 other life our globe can support. What determines how much plant 

 material is produced per earth per unit time? 



In principle, the amount of photosynthesis that takes place over 

 the world each year is ultimately limited by the efficiency with which 

 plants convert the energy of sunlight to energy stored as plant ma- 

 terial. This efficiency has been measured in many laboratory and field 

 experiments. In such a field experiment, the increment in plant sub- 

 stance per unit area and over a growing season is determined and its 

 energy content measured ( 1 gram of plant material releases on com- 

 bustion approximately 3.5 to 4.5 large calories). The amount of solar 

 energy incident on the unit area during the growing season is also 

 measured. Since only light in the visible region of the spectrum ( 4,000 

 to 7,000 A) is usefully absorbed by chlorophyll, we calculate photo- 

 synthetic efficiency on the basis of the amount of solar energy in this 

 wave-length region— approximately 50 per cent of the total. We now 

 calculate efficiency as the energy content of the plant material pro- 

 duced divided by the energy contained in the visible solar flux. This 



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