CHAPTER VI. 

 GROWTH. 



THE term growth not infrequently is used to imply mere increase 

 of the plant or plant member in various directions with little 

 or no attempt to correlate or to analyse these and other related 

 expressions of the activity of the organism. Thus increase in 

 surface is not necessarily growth : a pound pat of butter may 

 be spread over a number of slices of bread whereby its area is 

 increased but not its mass. An etiolated seedling may show 

 a much greater length of internode than its fellow of the same 

 age grown under normal conditions : but the comparison of 

 the dry weights of the two will show no increase in mass in the 

 etiolated example. 



From considerations such as these, the conclusion is 

 reached that growth is properly speaking an expression of the 

 metabolism of the organism ; it is, to use F. F. Blackman's 

 phrase, the finished product of the metabolic loom. It is this 

 aspect which will mainly be considered on the present 

 occasion. 



Metabolism has two sides, debit and credit : if the anabolic 

 processes are more intense than the catabolic, growth will 

 result ; if the catabolic processes are in the ascendent, decretion 

 obtains. From this it follows that the sure index of growth 

 is increase in dry weight, the credit balance of the two opposite 

 activities. Thus Boysen-Jensen * found that under maximum 

 illumination the carbon assimilation of Sinapis, a sun plant, 

 was 6 mgs. of carbon dioxide per 50 sq. cm. of leaf surface per 

 hour at 20 C., whilst the respiration at the same temperature 

 and for the same units of surface and time was -8 mg. of 

 carbon dioxide. This means that for an average plant of 

 Sinapis, the amount of dry matter made in a day in July is 



* Boysen-Jensen : " Bot. Tidsskr.," 1918, 36, 219. 

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