Charles Darwin (1880) the great evolutinoist had hypothesized the presence of growth substances in plants. In his book ‘The power of movement in plants’ Darwin has described the experiments concerning the growth movements in canary grass (Phalaris canariensis). He had noticed the unilateral bending of the shoot tip (Coleoptile) towards the source of light. Other significant observations made by Darwin on the phototropic responses in grasses are
(i) Seedlings grown in dark do not bend
(ii) Curvature occurs only when the tip is illuminated
(iii) Decapitating the shoot or covering the shoot tip with a light insulated material does not cause curvature.
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According to Darwin, when seedlings are exposed to a lateral source of light, some inducer is transmitted from the upper part to the lower part causing the curvature.
The following are some of the names concerned with the discovery of aunixns. Boysen Jensen (1910, 1911, 1913): Conducted significant experiments concerning the phototropic responses in Avena (oats) coleoptile. He found that decapitated tips do not exhibit curvature, but replacement of the tip on the cut end of the shoot reintroduces curvature under the influence of lateral light.
In another experiment he cut off the coleoptile tip of the seedling are replaced it with a thin plate of gelatin between the tip and the cut stem and observed that the coleoptile tip could still bend towards the source of unilateral light. Although he did not give any explanation, it was clear from his experiment that a material substance which can pass through gelatin acted as the stimulus for curvature.
Paal (1918) demonstrated that replacement of the coleoptile tip on the cut end of the shoot, unevenly would cause curvature towards the otherside eventhough there is no light (experiment being conducted in darkness) 1 his indicates the presence of a diffusible substance at the tip. F.W. Went (1917) finally provided the proof for the presence of a curvature inducing substance. He cut off the coleoptile tips and placed them on Agar blocks for some time.
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When these agar blocks were kept asymmetrically on the cut ends of coleoptile, there was curvature even in darkness. This experiment clearly showed that some substance from coleoptile tip diffused into agar blocks. Went named this substance auxin and opined that no growth can take place without these auxins.
The technique of asymmetrically keeping the agar blocks on Avena coleoptile tips to measure the auxin activity is the best bioassy for auxins and is popularly called the “Went Avena coleoptile test”.
Went (1928) also compared the quantity of the auxin in the illuminated and the non illuminated regions in the Avena coleoptile tip and found out that the shaded area has more (57%) auxin content than the illuminated area (28%). Auxins always move towards the base from the tip even if the orientation of the plant is reversed.