Leaves don't leap...

...they are pushed! In a phone conversation with my Dad yesterday, I remembered hearing about a scientist who proved that trees actively shed their leaves. I found the details on my 1st Google search. That scientist is decorated American botanist Peter Raven, and he has shown that the leaves of deciduous trees are not simply blown off by the wind, but severed by the tree. Triggered by chemicals activated by seasonal changes in light and temperature, special cells gather at the juncture of the leaf stem and the branch. They are called "abscission" cells, a word which has the same Latin root as the word "scissors." Within a few weeks, every leaf on the tree develops a microscopic bumpy line of cells (the red cells in the image below) that push the leaf, bit by bit, away from the stem and leaving it dangling. "So with that very slender connection, they're sort of ready to be kicked off," says Raven, who explains that the process is necessary to the tree's survival. If a tree kept its leaves permanently they would photosynthesize during warm weather all year round. Freezing with water in their veins would cause the leaves - and then the tree itself - to die. So the tree shucks its leaves in autumn as food production slows, and regrows them in spring as it speeds back up.