Plant Qrowth'Suhstances 



thetic growth-substances, Chailakhian's hormonal theory 

 (1937) of flowering may be mentioned. This worker pos- 

 tulated the existence of a factor for flowering, which he 

 named florigen. The mobility, and therefore the existence, 

 of such a hormone was demonstrated in several ways; for 

 example, grafts of Perilla on Perilla at different stages of 

 flower-ripeness were made, and the grafts behaved as if the 

 factor passed through the unions {cf. the grafting experiment 

 of Traub mentioned on p. 52).^ Practical applications of 

 hormone theory in horticultural practice are explained in the 

 1937 paper, which has some 300 references. By appropriate 

 techniques of vernalization [see below) transplanting, grafting, 

 and by taking advantage of photoperiodism, the concentration 

 of the flowering hormone can be controlled, and flowering 

 may thus be either forced or retarded. 



The vapour of methyl a-naphthalene-acetate has been 

 found by Guthrie (1939) to inhibit the growth of buds of 

 potato tubers. The substance is sufficiently volatile at a warm 

 room temperature (25° C.) to produce its effects. The sprout- 

 ing of whole tubers can be retarded by merely storing them at 

 that temperature in presence of paper impregnated with the 

 ester, e.g. in a bag. Epinasty of tomato leaves is readily 

 induced if a little of the substance is placed in a bell jar with 

 the plant. 



Zimmerman et al. (1939) discuss "Responses of plants to 

 growth substances applied as solutions and as vapours." 

 There are similarities between the effects of unsaturated 

 hydrocarbon gases (ethylene, etc. ; used for forcing) and those 

 produced by vapours of a number of "synthetics". 



Gustafson (1936) seems to have been the pioneer in show- 

 ing that known chemicals could cause the ovary of a flower 

 to develop into a fruit. By using indole-butyric acid he 

 obtained almost normal, but seedless, fruits from unfertilized 

 tomato flowers. A good set of (parthenocarpic) fruit was 

 secured by Gardner and Marth (1937-8) with holly and 



^In another paper Chailakhian and Zhdanova (1938) concluded that the 

 natural growth-hormones affect growth only, and do not supply the stimulus 

 for the formation of buds and flowers. 



5+ 



