3°° 



PHYSIOLOGY OF 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 1 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 

 pectinata) from which the tip has been 

 broken away for some time. One of 

 Fig. 144— Root system from which the the lateral branches is seen to have 



tip of the primary root has been cut away, become negatively geotropic and to 

 The laterals nearest to the cut have become . , . . .. . 



positively geotropic. {After Bruck.) have bent upward, just as if it were 



trying to replace the lost tip. 



Errera 2 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 the response impresses itself upon some 



Fig. 145. — Upper portion of tree of Abies peclinata. 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 complication of what may seem like purposeful- 



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

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



2 Errera, L., Conflits, de preseance et excitations inhibitoires chez les vegetaux. Bull. Soc. Roy. Bot. 

 Belgique 42: 27-43. 1904-1905. 



