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CHAPTER VI. 



GROWTH. 





The term growth not infrequently is used to imply mere in- 

 crease 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 in normal conditions : but the com- 

 parison 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. 



Metabolism has two sides, debit and credit : if the ana- 

 bolic processes are more intense than the catabolic, growth 

 will result ; if the catabolic processes are in the ascendant, 

 decretion results. From this it follows that the sure index 

 of growth is increase in dry weight, the credit balance of the 

 two opposing activities. Thus Boysen- Jensen * found that 

 under maximum illumination the carbon assimilation of Sina- 

 pis, a sun plant, was 6 mg. 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 0-8 mg. of carbon dioxide. This means that for an average 

 plant of Sinapis, the amount of dry matter made in one day 

 in July is 60 mg. whilst the loss of dry matter due to respira- 

 tory processes is 14 mg., leaving a balance of 46 mg., which 



* Boysen-Jensen : " Bot. Tidsskr.," 1919. 3<>, 219- 



169 



