Auxins - Plant Growth Substances
Julius Sachs (1882) first suggested on the basis of his observations that there are special substances for forming roots and some special substances for forming leaves etc. in plants (quite a far sighted hypothesis on the basis of strength of his scientific foresight). A decade earlier, Charles Darwin (known to all of us for his famous 'Theory of Evolution") and his son Francis studied the effect of gravity and unilateral light on the movement of plants. They demonstrated that the effects of light and gravity on the bending of both roots and shoots are mediated by the tip and that this influence can be transmitted to other parts of the plant.
Darwin was primarily interested in the coleoptile, which is a specialized leaf in the form of a hollow cylinder that enlcoses the epicotyl and is attached to the first node. It affords protection to the delicate growing tip of a grass seedling until, eventually the more rapidly first leaf emerges above ground. Paal and Hans Soding first approached this problem through experimental methods. They decapitated the coleoptile (removed the tip) and observed that this resulted in failure of coleoptiles to respond to light.