<P> Marcescence is the retention of dead plant organs that normally are shed . Trees transfer water and sap from the roots to the leaves through their vascular cells, but as the autumn season begins, these veins slowly close until a layer of cells called the abscission layer completely closes off the vein allowing the tree to rid itself of the leaf . Leaf marcescence is most often seen on juvenile plants and may disappear as the tree matures . It also may not affect the entire tree; sometimes leaves persist only on scattered branches . Marcescence is most obvious in deciduous trees that retain leaves through the winter . Several trees normally have marcescent leaves such as oak (Quercus), beech (Fagus) and hornbeam (Carpinus), or marcescent stipules as in some but not all species of willows (Salix). All oak trees may display foliage marcescence, even species that are known to fully drop leaves when the tree is mature . Marcescent leaves of pin oak (Quercus palustris) complete development of their abscission layer in the spring . The base of the petiole remains alive over the winter . Many other trees may have marcescent leaves in seasons where an early freeze kills the leaves before the abscission layer develops or completes development . Diseases or pests can also kill leaves before they can develop an abscission layer . </P> <P> Marcescent leaves may be retained indefinitely and do not break off until mechanical forces (wind for instance) cause the dry and brittle petioles to snap . </P>

What types of trees keep their leaves in the winter