METABOLISM DIGESTION TRANSLOCATION GROWTH 219 



underground and are not influenced by the sunlight. (3) 

 Tliigmotropism, which refers to the twining of the stems or 

 other organs of climbing plants ; friction, resulting from contact 

 of the sensitive organ of the climber with some other object being 

 the stimulant. (4) Hydrotropism, which refers to the move- 

 ment of organs, especially roots, towards moisture, which is the 

 stimulant. Note that geotropism and hydrotropism usually 

 work in combination. (5) Aerotropism, which refers especially 

 to the movement of roots into soil in which there is an abundance 

 of air, which is the stimulant. Aeration of the roots is a very 

 important factor in the growth of many plants, and explains 

 in part the necessity for cultivation of many of our crops. The 

 bald cypress, which grows best in swamps, produces peculiar out- 

 growths (knees) from the roots which rise into the air. (6) 

 Chemotropism, which refers to the growth movement of parts 

 towards or away from chemical compounds which may be useful 

 or injurious. This is well illustrated by the growth of the pollen 

 tube through the style and into the micropyle and also by the 

 growth of hyphse of fungi towards food. 



All these tropisms are growth movements and are restricted 

 to the young and growing parts of the plant. As the roots and 

 stems increase in age they become less and less mobile and finally 

 become immovable. When young plants are beaten down by 

 wind and rain storms, they will frequently resume their normal 

 positions in part or entirely; but when large trees are blown 

 down they cannot resume their original position. However, the 

 new growth will assume positions in accordance with the trop- 

 isms by which they are influenced. 



Relation of Plant Organs to Sunlight. It is well known 

 that plants growing in a window develop so as to bring the leaves 



