HORMONES 133 



is desirable even though the acquaintance be not cultivated 

 until some future occasion. 



For long it has been known that very small quantities of 

 various materials act as powerful stimulants ; the extraordinary 

 effect of minute traces of zinc on the growth of moulds may be 

 instanced. Animal physiologists recognize the effect of traces 

 of substances in stimulating various activities, especially those 

 associated with secretion. These substances are produced in 

 one organ and stimulate another organ to which they are con- 

 veyed by the blood. Hence the 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 has been adopted by Loeb f who concludes from 

 many observations on the development of buds and roots on 

 the leaves of Bryophyllum 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 hormone 

 and the mass of the lateral bud ; it is presumably for this 

 reason that the inhibition may only extend to these primordia 

 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 Ven- 

 kataraman \ can find no evidence in favour of the existence of 

 such bodies, and Child and Bellamy consider that inhibition 



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

 Bot.," 1911, 25, 507. 



fLoeb : " 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, I, 337. 



Child and Bellamy : " Science," 1919, 50, 362. 



