Download the clearing there is no such blanket of leaves overhead

88
Microclimates and vegetation
[Ch. 4
the clearing there is no such blanket of leaves overhead, so infra-red is radiated out to
the sky more easily.
4.1.6
Big plants ``make'' the microclimates of smaller plants
The plants that live on the forest ¯oorÐat low light levels, milder temperatures and
higher humidityÐare specialized to a microclimate made for them by the canopy
trees that absorb most of the sunlight. Their photosynthetic chemistry is specialized
to low light levels and they cannot cope with direct sunlight. These forest ¯oor plants
tend to have soft leaves, because leaves underneath the canopy have no need to be
``tough''Ðthey are not blown about by the wind, nor are they dehydrated in direct
sunlight. An example of one of these forest ¯oor plants is the African violet
(Saintpaulia), a common house plant which requires shade. As many houseplant
owners know all too well, it dies quickly when exposed to direct sunshine.
Some forest ¯oor plants have peculiar adaptations to help them gather as much
as possible of the light that falls upon them. Certain herbaceous plantsÐsuch as the
southeast Asian vine spike moss (Selaginella willdenowii ) and some species of Begonia
(Figure 4.4*)Ðhave a bluish sheen (known as iridescence) to their leaves. This is
caused by little silica beads within the epidermis of the leaf. Experiments have
suggested that these beads help the leaf to focus in light from a range of directions,
sending it straight into the photosynthetic cells below. In Selaginella each cell underneath a silica bead has a single large chloroplast which seems to be precisely located
to receive this focused beam of light.
The leaves at the top of a tree also make the microclimate for the leaves below
them. Even on the same tree, leaves that are out in full sunlight develop slightly
di€erently from those in the shaded branches down below. The ``sun leaves'' are
thicker with more layers of photosynthetic cells packed in, to take advantage of the
abundant light. The lacquer-like cuticle on the upper surface of a sun leaf also tends
to be thicker, to help reduce unnecessary evaporation. On a sun leaf there are more
stomataÐthe pores which open to let CO2 inÐso that the leaf can take advantage of
high light levels to bring in more CO2 for photosynthesis when it has enough water.
As soon as evaporation through the stomata becomes too intense and the leaf is in
danger of dehydrating, the stomata are clamped shut and the leaf relies on its cuticle
to prevent further water loss.
The chemistry and color of sun leaves also tends to be di€erent from shade
leaves. Shade leaves tend to be a darker green because they are richer in a particular
dark green form of chlorophyll (chlorophyll b) that is good at harvesting light at low
intensities and at the wavelengths ®ltered by leaves above. Sun leaves have more of
the chlorophyll a form which exploits high light intensities more e€ectively. The
upper epidermis of sun leaves is also packed with natural sunscreen compounds such
as ¯avenoids which absorb most UV light and prevent it from damaging the sensitive
photosynthetic cells below. Just putting a shade-grown tree seedling out into direct
sunlight shows how important this protection is: in a few days the shade-grown leaves
are bleached and useless.
http://www.springer.com/978-3-540-32491-1