Darwin was already interested in auxin in the 19th century. Only in recent years, however, has the hormone started to relinquish its secrets, thanks to intensive molecular research. Auxin is produced in the young, growing parts of plants and then transported throughout the plant -- to a low-lying stem for example. The stem needs to straighten out as soon as possible to be able to absorb the sun's rays efficiently; therefore more auxin will be delivered to the underside of the stem than to the topside, resulting in the underside growing faster and the stem straightening out. For the same reason, plants in front of windows will always turn to the light. This dynamic regulation of auxin transport allows plants to take optimal advantage of local and changing conditions.