Chapter Twenty-Four 



SOME SPECIAL CONSIDERATIONS OF 

 PLANT GROWTH 



Growth is an irreversible change in shape or weight of a 

 Hving organism. It usually means an increase in dry weight 

 because of the increase in cell number and size, but in case 

 of seedling growth the weight decreases (see table below) 

 because the stored food is being oxidized. Growth is the 

 result of many coordinated activities of the organism, some 

 of which are deeply involved in growth regulators and stimu- 

 lants. Some of our knowledge of hormones in relation to 

 growth is briefly described in Chapter 20. 



The germination in soil of radish and pea seedlings in full 

 sunlight, in 10 per cent full sunlight, and in darkness (Plate 

 XV) shows the external influence of light on growth. The 

 following table tells the story of the use of the food (in dry 

 weight) and the relative amounts of water in the green 

 weights: 



The leaves are thicker, the roots are better branched, and 

 the shoots are much shorter, with full sunlight. Shade has 

 little effect on the size of the leaves, but makes a marked 

 increase in stem growth, and a decrease in dry weight. The 

 dark-grown plants show the lack of leaf growth, the extreme 



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