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Photo 1: windowed
reticulated
leaves of
Haworthia emelyae
v. comptoniana

photo 2: A
longitudinal section showing the
transparent
tissues. |
Fenestrate
growth habit of some
subterranean leaf succulents.
A number of South African
succulents like Lithops ssp. and
Haworthia ssp. (See:
photo 1 and 2 ) to escape the
killing heat of the
desert
grow almost entirely
subterranean and have
evolved special
fenestrate leaves that
grow below the
surface of the soil;
This succulent
leaves have a nearly
transparent flat
apex
exposed to the
atmosphere and are even
at the soil level. This
permit the plants to minimize the
heating effects of high
light intensity and to reduce the
drying effects of
desert
air during
prolonged drought. There are no
stomata in the windows, so
little water is lost, these plants remain
subterranean because
they have contractile roots which continually pull the plants
deeper into the ground as the
stem
elongates.
The disadvantage, however, is the limited amount of
leaf
surface
area exposed to the
sun for
photosynthesis to supply
food for
the plant but the translucent window in the top
surface,
however, the fenestration allow
sunlight to be captured and utilized even when
the plant are retracted below the
surface of the soil. This is
an interesting evolutionary
adaptation to overcome this
light
problem; their leaves are a simple optical system that permits
light striking the windows to be
diffused onto the green,
photosynthesizing surface situated
underground. So, with a minimum
of exposure to the outside
environment, a maximum area of
photosynthetic
tissue is ensured.
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