INTERRELATIONS BETWEEN PARTS OF PLANTS 397 



tops, which previously grew obhquely, now begin to grow 

 vertically. 



Such instances of reciprocal influence of parts of the plant on 

 the character of growth have been generally termed ''correla- 

 tions." The nature of these phenomena for a long time remained 

 doubtful and this gave occasion for different sorts of vitalistic 

 interpretations. It may be assumed that correlations are 

 phenomena of a hormonal character, i.e., that growth is regu- 

 lated by means of specific chemical agents. According to this 

 concept, the top of the stem elaborates special substances 

 that inhibit cell division in the meristematic tissues. Thus 

 tbey likewise retard the growth of the lateral buds and retain 

 them in a ''dormant" state. With the removal of the top, the 

 stream of this inhibiting substance is checked, and the lateral 

 buds begin to develop rapidly. According to recent data, this 

 substance is identical with the growth-promoting substance 

 auxin (see Art. 29). 



An analogous correlation may be observed in the development 

 of the root system. The growing root tip inhibits the formation 

 of laterals near the tip, and branching always begins at a con- 

 siderable distance from the end. If the root tip is removed, 

 immediately an acceleration in the initiation and development 

 of the laterals will take place. This phenomenon is well known 

 in the practice of horticulture and vegetable gardening. Trans- 

 planting seedlings serves not only to give each plant a larger 

 root surface but also changes the character of the growth of the 

 root system. In transplanting, the main root may be cut off 

 and replaced by lateral rootlets, which spread into the surface 

 soil layers. Such a structure of the root system later on facili- 

 tates further transplanting of the seedlings from the beds into 

 the field. ^ 



The method of starting crop plants by means of transplanting 

 of seedlings is very ancient and is chiefly applied in severe 

 climates. The seedhngs are usually raised in hotbeds or green- 

 houses during the colder season early in the spring, when there 

 is sufficient fight for successful nutrition and growth of plants, 

 but the temperature in the open is too low. With the beginning 

 of the warmer season, the seedlings are transplanted into the 

 field. This allows a considerable increase in the length of the 

 vegetative period; and by this procedure, such southern crops 



