20 



GROWTH 



In the simpler organisms the rapidity of growth under uni- 

 form external conditions may be almost constant, but with the 

 more complex plants and animals growth varies in rate with age. 

 This is a familiar fact. Young animals and young plants grow 

 rapidly, but as they approach maturity growth of the plant or 

 animal as a whole ceases, although some growth probably con- 

 tinues in some parts as long as the individual remains alive. 

 Even in man growth of new blood corpuscles and the growth 



of new tissue to cover a wound 

 occurs in the aged, and hair 

 and finger nails may grow 

 somewhat after death. The 

 curve in Figure 10 illustrates 

 the cessation in growth in 

 length as the sunflower be- 

 comes mature. Growth of man 

 plotted in the same way would 

 produce much the same kind 

 of curve though the units 

 would be different. What 

 causes this limitation in growth 

 is a very interesting question. 

 Why don't we continue grow- 

 ing forever? In annual plants such as the sunflower the cessation 

 of growth is associated with the onset of reproduction — forma- 

 tion of flowers, fruits, and seeds. In perennial woody plants 

 eventual cessation of growth in height is partly due to difficulties 

 incident to supplying sufficient water to the top of the tree as 

 well as to other causes. In animals the problem is a very com- 

 plex one. There is apparently no inherent reason why an animal 

 should cease growing, because tissues, for example those of the 

 heart, which cease growth when they reach normal size in an 

 animal body, may be kept growing indefinitely if removed 

 while young from the body and cultivated in a sterile fluid. 

 This is called tissue culture. The development of toxic mate- 



Figure 10. The height of the sunflower 

 plant, Helianthus, from planting to ma- 

 turity. Note that growth in height ceases 

 as the plants approach maturity. After 

 Reed and Holland. By permission of Na- 

 tional Academy of Sciences. 



