HORMONES, VITAMINS, AUXIMONES 209 



tions of nervous control are now generally accepted but they 

 clearly show how our attitude toward nervous activity has changed 

 in the past few years.' ' 



Hormones have not been isolated from plants or demonstrated 

 so clearly as in animals, but their presence is inferred to explain 

 the physiological changes mentioned and described below, which 

 are otherwise inexplicable. 



Thus Errera (1904) explained by means of internal secretions 

 the changes in geotropism observed in spruce branches. The 

 leader or central axis of a spruce is negatively geotropic while 

 the lateral branches are diageotropic ; but if the leader is removed, 

 one of the lateral branches will then become negatively geotropic 

 and grow up as though trying to replace the lost tip and carry on 

 the functions of the leader. According to Errera, the apical shoot 

 secretes an inhibiting substance which keeps the other branches 

 diageotropic in their normal position, but when the main tip is 

 removed, the source of the inhibiting hormone is likewise removed 

 and a lateral shoot then assumes the functions and characteristics 

 of the lost leader. 



Bruck (1904) observed the same response in roots. When the 

 terminal two millimeters of the primary root is removed, the 

 lateral roots just above become positively geotropic and bend 

 downward to replace the removed organ. 



Vochting (1908) showed that in rape (Brassica rapa) when the 

 blossoms were removed the uppermost leaf became vertical 

 instead of lateral as normally. Similarly Bassler (1909) showed 

 that in many annuals which bear no branches, the cutting off of 

 the stem results within twenty-four hours in an upward bending 

 of the leaves nearest the cut of from five to thirty degrees or even 

 more. Those nearest the cut bend up the most. All these exam- 

 ples can be explained in the same manner; and further support is 

 given the hormone hypothesis by the fact that a vertical cut, 

 which would not interfere with the transport of the hormone, 

 does not cause the response by the lateral leaves or branches. 



Similarly Loeb (1915-1924) concluded that the development 

 of buds and roots on the leaves of the live-forever (Bryophyllum 

 calycinum) was prohibited by a hormone secreted by the apical 

 bud, since the buds lower down on the edge of the leaf will not 

 develop until the apical bud is removed. The removal of this 

 apical bud shuts off the flow of hormones from it. Since the bud 



