The phenomenon of rapid defoliation in the fall is one which may be often observed in our common trees. The causes at work are not a little mysterious, but some understanding of them may be gained by considering the method of formation of the separating cork-plane. This cork-plane begins to be formed sometimes as early as the summer, and by autumn extends almost entirely across the leaf-stalk. All that is needed to insure complete severance is the formation of a few more cells, and the ripening of the whole into a readily separable condition. A sharp frost seems to hasten this final process in a very effectual manner, for, so far as we have observed, it is just after a "cold snap" that we have the most marked defoliation. The exact way in which sudden cold acts to put all parts in readiness for the separation is not clear, but it may be fairly surmised that its effect i is felt not so much in the production of new cells as in the ripening of those already formed. All who have prepared specimens for an herbarium know that, with many plants, the leaves separate from the plant spontaneously during drying.

This shows clearly that cold is not necessary to the process of defoliation, and leads us to make the suggestion that it may be a dryness that comes with the cold, rather than the temperature itself, which gives the final touch. - F. L. Sargent, in Popular Science News.