Why do most annual plants die in the autumn?  Larry D. Nooden and Susan L. Schreyer at the University of Michigan are studying a chemical "death signal" possibly a hormone which they have traced to plant seeds.  The possibility is being considered that seeds inside mature fruits such as soybean pods send out hormones, which cause plants to yellow and die even before nights cold enough for freezing cut them down.

Gardeners for years have known that if faded flowers are picked before they form seeds the plants will continue to produce more flowers.  Pansies are a  good example. Among the vegetables okra will continue from early spring to frost if the pods are kept picked before they harden.

Nooden says that this idea was tested on soybeans. Growing pods were plucked from one side of the plant only and allowed to remain on the other. The side with the mature pods and seeds turned yellow and died, the other remained healthy.