PHOTOTROPISM EXPERIMENTS

=Phototropism - Experiments= [image:http://i.imgur.com/bhr4U9t.png?1] Some of the early phototropism experiments were conducted by Charles Darwin (best known for his contributions to evolutionary theory) and his son. He noticed that if light is shone on a coleoptile (shoot tip) from one side the shoot bends (grows) toward the light. The ‘bending’ did not occur in the tip itself but in the '''elongating''' part just below it. Removing the tip or covering it with foil meant that the shoot could no longer ‘bend’ toward the light. Covering the elongating part of the shoot did not affect the response to light at all! Darwin Concluded that: “Some influence is transmitted from the tip to the more basal regions of the shoot thereby regulating growth and inducing curvature” [image:http://i.imgur.com/5qfPbpd.png?1] Boysen-Jensen continued investigating these responses. He cut the tips off coleoptiles and placed a thin piece of silver or mica between the coleoptile and the lower shoot. The result was that the shoot did not grow or curve toward the light. When he repeated the experiment using a block of gelatin / agar instead, the result was that the shoot grew and curved towards the light. Thus he concluded that the Darwin’s ‘influence’ was a water soluble chemical, capable of diffusing through the agar / gelatin from the tip where it was produced to the lower, elongating part of the shoot where it had its effect. This water soluble '"influence" was later identified as the plant hormone auxin. [image:http://i.imgur.com/b7rkB5N.png?1] He also noticed that placing a thin piece of mica part way into the dark side of the coleoptile blocked any response whereas placing a thin piece of mica part way into the light side of the coleoptile did not affect the response. [image:http://i.imgur.com/roBZTRm.png?1] A third scientist (Went) cut the tips of coleoptiles and placed them in the dark, putting a few of the tips on agar (gel) blocks that he predicted would absorb the growth-promoting chemical. He then placed these blocks on top of the tip-less coleoptiles. When the agar block was centred on top the coleoptile grew straight. If the agar block was offset, resulting in an uneven distribution of the chemical on one side, the shoot would curve as though it was growing towards a light source. This proved that the response was due to a water soluble chemical that diffused from the tip of the plant down the dark / shaded side of the coleoptile causing it to curve towards the light. Went repeated the experiment with agar that had not been treated, which produced no growth. This is known as a ''control'' and is used as a comparison / to check that it was a chemical within the agar that was responsible and not the agar itself.