GROWTH OF PLANTS 143 



affair than it appeared at the outset. The conditions under 

 which the experiments succeed have not as yet been determined. 



Among the chemical stimulants, carbon dioxide, the normal 

 product of respiration, occupies a singular position. Its accumu- 

 lation in the atmosphere surrounding the growing organs causes 

 a retardation of their growth and later checks various other vital 

 processes, which after some time may be renewed. Thus, 

 under the influence of carbon dioxide, the plant passes into a 

 dormant state. To increase their storage life, fruits and vege- 

 tables are placed in an atmosphere of carbon dioxide in closed 

 containers. This checks the development of molds, the chief 

 enemies of fruit during storage, and retards respiration. These 

 storage methods are applicable however only to such organs of 

 plants as possess a low vital activity, such as fruits and vege- 

 tables. This method is of commercial value in the storage of 

 apples and in transporting berries to the market. Vigorously 

 growing parts of the plant, e.g., germinating seeds, succumb very 

 quickly in an atmosphere of carbon dioxide with symptoms of 

 autointoxication from products of their vital activity. 



34. Influence of Gravity on the Position of Growing Organs. 

 Geotropism. — Independent of the position occupied by the seed 

 in the soil, the seedling always develops its root vertically down- 

 ward and its stem upward. Thus the root descends into the 

 soil and begins to absorb water and nutrients, while the stem 

 rises to the surface and presents its leaves to the sun's rays, 

 which play the main part in the formation of organic substances 

 by the plant. This orientation of the organs in relation to the 

 force of gravity is retained throughout the life of the plant. 

 The main axis of both stem and root grow vertically, the lateral 

 shoots and roots grow at some definite angle in relation to the 

 main axis, while the leaves are usually adjusted in a more or less 

 horizontal plane depending on the direction of light. If the 

 plant is disturbed -from this orientation by some external influ- 

 ence, e.g., if it is bent toward the ground by wind, either the 

 entire plant rises, as may be observed in cereals after ''lodging," 

 or the younger parts curve so that the newly developing organs 

 again assume a normal orientation with respect to gravity. This 

 latter form of reaction is found in most plants. 



The production of such curvatures leading to a restoration of 

 the normal orientation of plant organs is closely connected with 



