268 



PHYSIOLOGY OP GROWTH AND CONFIGURATION 



The following experiments show that gravitation acts only as a release, the 

 conditions that control the phenomena of geotropic response residing in the 

 plant itself. As has been mentioned, lateral roots do not exhibit positive, geotro- 

 pism, but are diageotropic, taking a position nearly horizontal when the axis of 

 the plant is vertical. Bruck^ has shown, however, that when the terminal 2 mm. 



of the primary root is cut away, thus 



putting an end to the elongation of this 



organ, then the laterals just above the 



wound become positively geotropic and 



bend vertically downward (Fig. 144). 



The same sort of response is observed 



in stems. In Fig. 145 is shown the 



upper portion of a fir-tree {Abies 



pedinata) from which the tip has been 



broken away for some time. One of 



the lateral branches is seen to have 



become negatively geotropic and to 



have bent upward, just as if it were 



trying to replace the lost tip. 



Errera^ proposed to explain such phenomena as those just mentioned by 



postulating "internal secretions;" that is, special hormones that might regulate 



growth. In those cases where lateral roots or shoots take the place of primary 



ones the apparent purposefulness of Ihe response impresses itself upon some 



Pig. 144. — Root system from which the 

 tip of the primary root has been out away. 

 The laterals nearest to the cut have become 

 positively geotropic. (After Bruck.) 



Fig. 145. — Upper portion of tree of Abies pedinata. RemoVal of the tip of the main stem has 

 made one of the branches negatively geotropic. (After Errera.)' 



minds so strongly that it is not easy for them to think of the chemical basis of 

 the phenomena in question. There are other cases, however, where similar 

 responses occur without the comphcation of what may seem hke purposeful- 



1 Bruck, Werner, F. Untersuchungen iiber den Einfluss von Aussenbedingungen auf die Orientierung der 

 Seitenwurzeln. Zeitsch. Physiol. 3: 486-518. ' 1904. 



^ Errera, L., Conflits de pr£s6ance et excitations inhibitoires chez les v^gfitaux. Bull. Soc. Roy. Bot. 

 Belgique 42 : 27-43. 1904-1903. 



