LIGHT AND MOVEMENT 41 



fundamental step was due to Boysen-Jensen (1910-13) who showed that the 

 stimulus could trav^erse a layer of gelatine but was arrested by a plate of mica, thus 

 demonstrating that the curvature was due to the diffusion down the shaded 

 side of the plaiit of a chemical substance stimulating growth (Fig. 17). These 

 observations were confirmed by Paal (1914^18) who showed, moreover, that if 

 an unstimulated tip were excised and replaced towards one side of the stvunp, 

 growth was accelerated on that side, thus demonstrating that the stimulatory 

 substance was continuously formed in the sensitive region. The final proof 

 was effected by Stark (1921), Stark and Drechsel (1922), Cholodny (1927-35) 

 and especially by the Dutch botanist, Went (1926-45), who trapped the diffusible 

 growth-hormone descending from the coleoptile tip in a piece of gelatine or 

 agar inserted into the plant and, transferring the jelly from the plant and placing 

 it on the cut end of a non-illuminated plant from which the tip had been removed, 

 demonstrated the occurrence of a typical jahototropic response in the second 

 even although light had been entirely excluded (Figs. 18-20). All that remained 

 was to identify the chemical nature of the active agent. 



A growth-hormone of this type was first extracted from fungi by Nielsen 

 (1930) and Boysen-Jensen (1931), and shortly thereafter was chemically 

 identified by Kogl (1932) and Kogl and Kostermans (1934) as 3,indole-acetic 

 acid. Subsequent intensive research, particularly by Kogl and his colleagues 

 (1931-35) in Germany, Zimmerman and Hitchcock and their colleagues (1935-48) 

 in the Boyce Thompson Institute for Plant Research in New York, has shown 

 that there are many such physiologically active substances {auxins) of wide 

 distribution ; indeed, over 50 compounds, natural and synthetic, having this 

 growth-producing property had been isolated by 1935. The most interesting 

 historically are auxin a (a monocyclic trihj^droxy-carboxylic acid, Cj^8H3205), 

 auxiyi h (a monocyclic hydroxy-keto-carboxylic acid, CjgHgoO), and heterauxin 

 (3,indole-acetic acid, C^oHgOaN) (Kogl, 1935). Whether the first two or other 

 allied substances are present in the living j^lant • is not clear ; but the most 

 popular hypothesis at present is that heterauxin is present in the tip of the stem 

 initially as a precursor ; here it is activated into freely moving auxin by enzymic 

 action ; and it would appear that its activity may be masked or reduced by 

 anti-auxins. However that may be, it is clear that svich substances applied to 

 the intact plant or inserted into incisions or fed to the plant through the soil not 

 only induce tropic curvatures but can modify the plant in size, shape, pattern 

 and texture, can inhibit the formation of buds and perhaps of flowers, ^ and in 

 supra-physiological concentrations can induce tumour-like growths. ^ It is 

 puzzling why the same substances are found in human saliva (Seubert, 1925) 

 and urine (Kogl and Smit, 1931). 



It is interesting that an artificial end-organ to stimulate phototropic 

 activity can be synthesized (Brauner, 1952). If capillary tubes filled with photo- 

 sensitized indolylacetic acid are svibstituted for the cotyledons in Helianthus 

 seedlings, illumination of one produces a marked curvature of the other hypocotyl. 



This description may give the impression of over-simplification. It must 

 not be thought that the whole story of the growth of plants is explained in 

 terms of a single auxin. Research in progress as this book is being written is 

 showing that the regulation of growth is based on a complex system of several 

 auxins, kinetin-like hormones and gibberellin-like hormones, and possibly 

 other related substances. 



1 p. 12. 



^ For general reviews, see Boysen-Jensen (1936),^Went (1939), Zimmerman (1948). 

 van Overbeek n956), Bentley (1957). 



