60 PHYTOHORMONES 



however, appear to be based on insufficient experimental 

 data. 



As to the effect of external factors, du Buy (1933) found 

 that at low as well as high temperatures auxin production 

 is reduced, the optimum being around 25°. Gravity, irre- 

 spective of its direction of action, has no effect on auxin 

 production (Went, 1926; Dolk, 1930; Pfaeltzer, 1934). The 

 effect of light, both as regards wave-length and intensity, 

 has given very variable results, some of which are conflicting; 

 a systematic study would be very desirable (c/. Went, 1928; 

 du Buy, 1933). The effect of light on auxin production in 

 more fully grown plants is somewhat better understood 

 (see below). 



Some knowledge about auxin production may be derived 

 from growth measurements. Thus, the experiments of 

 Oosterhuis (1931) indicate that in Asparagus the growth of 

 the stem is largely controlled by the terminal and axillary 

 buds; these, therefore, are presumably the auxin-producing 

 centers. Similarly, Uyldert (1931) showed that, in Trades- 

 cantia, the apex, consisting of the bud and one leaf, promotes 

 the growth of the lower internodes, and hence she regarded 

 the apex as an auxin-producing center. 



In Helianthus seedlings, Fliry (1932) found that auxin 

 was formed in the plumula, but not in the cotyledons. The 

 lack of production in the cotyledons agrees with the findings 

 of Beyer (1925). Navez (1933a) found that the cotyledons 

 and plumula of Lupinus seedlings also produce auxin, and 

 further that seedlings grown in light produce about twice 

 as much as those grown in the dark. In the seedlings of 

 Lepidium and Raphanus van Overbeek (1932, 1933) showed 

 that there is auxin production both in the plumula and the 

 cotyledons, but that production in the cotyledons soon falls 

 off unless the plants are exposed to light. Dijkman (1934) 

 obtained auxin from all parts of Lupinus seedlings grown 

 in the dark, and concluded, in disagreement with the work 

 just mentioned, that there was no special auxin-producing 

 center in these plants. 



