212 GROWTH 



idea of a chemical messenger, or, to use the current term, 

 hormone. 



Errera * was amongst the first of botanists to suggest that 

 hormones play a part in the economy of the plant ; his ex- 

 planation of the changes in the spruce instanced above is that 

 the apical shoot continually is secreting an inhibiting sub- 

 stance which is distributed to other parts of the plant, keeping 

 them in their normal tone. The removal of the leader of the 

 spruce removes the source of the inhibiting hormone, where- 

 fore a near lateral shoot assumes the qualities and functions of 

 the lost leader. 



This idea was adopted by Loeb f who concluded from 

 many observations on the development of buds and roots on 

 the leaves of Bryophyllmn calycinum that the apical bud 

 secretes a hormone which inhibits the development of buds 

 more basal in position. It is not until the apex is removed 

 that the buds below will develop. 



The degree of inhibition depends on the amount of hor- 

 mone and the mass of the lateral bud ; it is presumably tor 

 this reason that the inhibition may only extend to these pri- 

 mordia situated more immediately below the apex. 



This account, brief though it be, will give some idea of the 

 action of hormones ; the hypothesis, in so far as it affects 

 plants, however, is not universally accepted. Thus Fyson 

 and Venkataraman % can find no evidence in favour of the 

 existence of such bodies, and Child and Bellamy § consider that 

 inhibition is a question of the conduction of stimuli rather 

 than the movement of tangible substances. Indeed, the 

 evidence regarding the occurrence of hormones in plants is, 

 perhaps, less conclusive than the proof of their existence in 

 the animal ; actually, hormones have never been demonstrated 

 in the plant ; their presence is inferred in order to explain 



* Errera : " British Ass. Rep.," 1904, 814. See also Armstrong : 

 " Ann. Bot.," 1911, 25, 507. 



f Loeb : " Bot. Gaz.," 1915, 60, 249 ; 1916, 62, 293 ; " Science," 

 1917, 46, 547. See also Reed and Halma : " Plant World," 1919, 22, 239. 



% Fyson and Venkataraman : " Journ. Indian Bot.," 1920, 1, 337. 



§ Child and Bellamy : " Science," 1919, 5<>, 362 ; " Bot - Gaz.," 1920, 

 7°t 249. 



