Leaves should be viewed as being temporary structures that are expendable to
the plant and to be replaced with the new growth. In some xerophytic plants,
the leaves may be highly reduced in size or modified into spines.

Then there are the leaf succulents. They are usually not deciduous the way a
maple tree is. The leaves are modified to carry on photosynthesis in an excess
of light and heat. There are various mechanisms for the plant to replace old
leaves in an orderly fashion, the most remarkable being Lithops, which grow
their new leaves inside the old ones.
Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)