INTRODUCTION 



21 



rial, the exhaustion of some essential growth material, and 

 other hypotheses have been offered. The problem is further 

 complicated by the fact that in most vertebrate animals there 

 are more or less sharply marked growth cycles, periods in which 

 growth speeds up and then slows down. 



Heredity is an important determiner of growth in both plants 

 and animals. A mouse will never grow to be as large as an ele- 

 phant even if we feed it on 

 hay and peanuts. In Figure 12 

 we have a striking example of 

 heredity as a determiner of 

 growth. The plant on the left 

 is a dwarf corn plant, the one 

 on the right is a plant of the 

 common type of corn. They 

 have grown side by side in a 

 field. Nothing we can do with 

 the dwarf plant will make it 

 grow higher. By limiting the 

 water, mineral salt supply, 

 light, and other factors in the 

 external environment we could 

 limit the height which the 

 normal plant would make to 

 that of the dwarf. The effect 

 of external factors such as those mentioned above on the growth 

 of plants is nowhere more strikingly shown than in the dwarf 

 trees produced by Japanese gardeners, who grow in a flower pot 

 a pine tree eighteen inches high but two hundred years old. 



The effects of water, temperature, and toxic materials are 

 more marked in their effects upon the growth of plants than 

 they are on most animals. The water supply is particularly im- 

 portant for the growth of plants. This is because increase in the 

 size of cells in plants is largely due to absorption of water (in 

 animals it is largely due to increased construction of proto- 



Figure u. Growth of an isolated corn root 

 tip. The root tips are maintained under 

 sterile conditions in a nutrient solution. The 

 one on the right is supplied sugar. The one 

 on the left has none. This is a "tissue cul- 

 ture" experiment with a seed plant. After 

 Robbins. By permission of the University 

 of Chicago Press. 



