Autumn/Winter 2011/12 - Harcourt Arboretum - University of Oxford

Botanic Garden News | No. 79
3
into the UK unintentionally with plants
before the 1920s. The tiny 2mm-long weevil
Stenopelmus rufinasus has been here for
decades and yet has found no other plants
palatable. It is therefore now acceptable to
release large numbers of the weevil even
on Sites of Special Scientific Interest (SSSI).
Using one introduced organism to control
another is a risky business but in this case it
seems to be safe.
Azolla is odd. It is a fern, as already
mentioned, but it does not produce clouds
of spores from the underside of its leaves, as
do bracken and the like. The floating green
plants of Azolla produce a structure found in
very few ferns – a sporocarp. This is a vessel
which contains the sporangia that produce
the spores. (The rusty pustules on the
underside of terrestrial ferns are sporangia.)
However Azolla, unlike most ferns, produces
two types of sporangia, male and female,
and so two different sporocarps. The female
sporocarps are smaller but they contain
just one sporangium which produces just
one female spore. When conditions are
right, the spore germinates and grows into
the female gametophyte inside the spore
but this then bursts through the spore wall
Azolla on the River Cherwell in Oxford
revealing several archegonia each with one
egg. (In case it has slipped your mind, the
gametophyte is a stage in the life of plants
that has no equivalent in the life of animals
and fungi. The gametophyte is responsible
for producing gametes: sperm in antheridia
and eggs in archegonia.)
The male sporocarps are 2mm in
diameter and contain many male sporangia
which each produce many male spores
which, despite being released from the male
sporocarps, hang together in a mass called a
massulae. The spores have a barb (a glochidium)
and this also helps the group of spores
hang on to a female spore that contains the
female gametophyte. Inside each male spore
a male gametophyte develops, upon which
develops one antheridium that produces
just eight sperm. These swim to the archegonium
and fertilise the egg. The resulting cell,
the zygote, divides and grows into the green
Azolla plant and the life history is complete.
Despite its current dubious reputation
Azolla has a past that might make it important
in the future. A long, long time ago in
the Eocene, 55.8 to 37.2 million years ago,
the continents were distributed differently
from their current arrangement. In particular
there was an ocean on the North Pole that
was almost completely surrounded by land
and thus cut off from the deep, global ocean
currents. At this time, the Earth was so warm
that the flora near the poles was what we
now consider to be sub-tropical.
The isolation of the Arctic Ocean and the
high temperatures led to an odd, layered
arrangement of the water in the ocean
similar to that found in the Black Sea today.
This consisted of a layer of relatively warm
freshwater (from the rivers surrounding
the Ocean) on top of colder and denser salt
water. This was particularly so around the
edge where the water was also enriched with
minerals such as phosphorous washed in
by the rivers. These were perfect conditions
for Azolla, which grew like Topsy, taking up
vast amounts of carbon dioxide. It has been
calculated that over an 800,000 year period
the Azolla, which covered up to four million
square kilometres, caused an 80% drop in
carbon dioxide levels from 3,500 ppm to
650 ppm.
As the Azolla died and sank in the waters
its decay was halted and the carbon was not
released. It was then even more securely
locked away on the bed of the ocean in
sedimentary layers. This huge decline in
carbon dioxide levels started the chain of
events that led to the present global climate
with ice at the poles rather than palm trees.
Perhaps this could be considered in the
search for a method of sequestering and
removing carbon dioxide today We think
Azolla is an invasive thug, but perhaps it
could be our saviour As I said at the beginning,
biology is rarely black and white.
Azolla with sporocarps