PHOTOTROPISM 



Kenneth V. Thimann and George M. Curry^ 

 Hm-vard Biological Laboratories, Cambridge, Massachusetts 



Phototropism stands near the base of a pyramid of light-reactions 

 of increasing complexity, the apex of which is vision. Yet it is not 

 quite at the base, for a still simpler reaction is light-chrected free 

 movement, shown by purple bacteria and motile cells of many aqua- 

 tic plants, as well as free-swimming stages of many invertebrates. 

 Such free movement, though sometimes erroneously referred to as 

 phototropism, is properly called phototaxis. Phototropism is the 

 light-directed curvature of fixed organisms (nearly always plants) . 

 It was Charles and Francis Darwin, as long ago as 1880, who showed 

 that some of the most phototropically sensitive plants were the seed- 

 lings of the Gramineae, which have served as the major experimental 

 material ever since. Among many other things, the Darwins showed 

 that in these plants the zone of light perception was the tip of 

 the coleoptile, while it was the part some millimeters below the tip 

 which responded by curving. The recognition that the zones of per- 

 ception and response are separated in space led to a sequence of 

 studies by Rothert, Boysen Jensen, Paal, Soding, Cholodny, and 

 Went, which established that the growth of the coleoptile is con- 

 trolled by a growth hormone, termed Auxin. This substance is pro- 

 duced in the tip and transported thence to the lower parts, which re- 

 spond by elongating. The logical consequence of such a concept is 

 that when the plant curves, instead of growing straight upward, 

 the curvatiux' is due to an excess of auxin on one side, causing that 

 side to elongate faster than the other side. If curvature takes place 

 towards the light {positive phototropism) this would mean that an 

 excess of auxin nuist occur on the side away from the light. No 

 sooner had this theory, applicable to all tropisms, been proposed 

 by Cholodny (1927) than it was confirmed by Went (1928). Went 

 allowed the auxin coming from the tip of an illuminated oat coleop- 

 tile to diffuse out into two small agar blocks in contact respectively 

 with the lighted and shaded sides of the plant. On assaying the 

 auxin activity, by a]>plying these blocks to the sides of other coleop- 



' I'lcsent address: 1 tilts I iiivtisity, Medford, Mass. 



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