Evolution of leaves

The lycopod Isoetes bears microphylls with a single
vascular trace.Leaves today are, in almost all
instances, an adaptation to increase the amount of
sunlight that can be captured for photosynthesis. Leaves
certainly evolved more than once, and probably
originated as spiny outgrowths to protect early plants
from herbivory.

The rhyniophytes of the Rhynie chert comprised nothing
more than slender, unornamented axes. The early to
middle Devonian trimerophytes, therefore, are the first
evidence we have of anything that could be considered
leafy. This group of vascular plants are recognisable by
their masses of terminal sporangia, which adorn the ends
of axes which may bifurcate or trifurcate. Some
organisms, such as Psilophyton, bore enations. These are
small, spiny outgrowths of the stem, lacking their own
vascular supply.

Around the same time, the zosterophyllophytes were
becoming important. This group is recognisable by their
kidney-shaped sporangia, which grew on short lateral
branches close to the main axes. They sometimes branched
in a distinctive H-shape. The majority of this group
bore pronounced spines on their axes. However, none of
these had a vascular trace, and the first evidence of
vascularised enations occurs in the Rhynie genus
Asteroxylon. The spines of Asteroxylon had a primitive
vasuclar supply - at the very least, leaf traces could
be seen departing from the central protostele towards
each individual "leaf".