Phototropism

Plant physiologists have investigated phototropism for over 100 years. The best known early research on phototropism was by Charles Darwin, who reported his experiments in a book published in 1880, The Power of Movement in Plants. Although Darwin was better known for his earlier books on evolution (The Origin of Species and The Descent of Man), this book was an important contribution to plant physiology.

Darwin studied phototropism in canary grass and oat coleoptiles. The coleoptile is a hollow sheath of tissue which surrounds the apical axis (stem) of these and other grasses. Darwin demonstrated that these coleoptiles are phototropic in that they bend toward a light source. When he covered the tips of the coleoptiles, they were not phototropic but when he covered the lower portions of the coleoptiles, they were phototropic. Darwin concluded from these and other experiments that (a) the tip of the coleoptile is the most photosensitive region; (b) the middle of the coleoptile is responsible for most of the bending; and (c) an influence which causes bending is transmitted from the top to the middle of the coleoptile.

The Dutch-American botanist Frits Went built upon Darwin's studies and began his own research on phototropism as a student in the 1920s. In particular, Went attempted to isolate the chemical influence which Darwin described. He took tips of oat coleoptiles and placed them on small blocks of agar, a special type of gel. Then, he placed these agar blocks on the sides of other coleoptiles whose tops he cut off. Each coleoptile bent away from the side which had the agar block. Went also performed important control experiments. He observed that plain agar blocks which were placed beneath the lower portions of coleoptiles had no effect on coleoptile bending. Went concluded that the coleoptile tips contained a chemical substance which diffused into the agar blocks Plants respond to the direction and amount of light they receive. The seedling at the right was grown in normal, all-around light. The one in the center received no light. The plant at the left grew toward the light that it received on only one side. Photograph by Nigel Cattlin. Photo Researchers, Inc. Reproduced by permission. and he named this substance auxin. The auxin which Went studied was subsequently identified by chemists as indole-3-acetic acid (IAA). IAA is one of many plant hormones which control a number of aspects of plant growth and development.