Commonly called the Tree Lupin, this plant is often found living in the hot, dry, sandy
shore ecosystem or as a secondary coloniser in exposed dry soils after clearing of other
vegetation. In the sandy shore ecosystem, colonisation of the bare sand begins with marram
grass, followed by lupins.
These lupins, living in sand, need to be adapted for both water conservation and the
ability to withstand exposure to high temperatures. Summer temperatures on the sand are
often in the 50-60 degree Celsius range. Winds blow beach sand to produce a highly
Lupins are plants which live in, and are well adapted to dry conditions. Many of their
adaptations are concerned with water conservation.
Structure and adaptations
These grow rapidly so that the young seedling will be able to quickly reach water, this
is of obvious importance in fast draining sandy soils. Seedlings germinate in the late
winter while the upper layers of sand still contain some moisture. As the root matures it
develops a bark-like layer which helps prevent desiccation.
Root nodules are bump-like growths on the side of the roots. They develop soon after
germination and are important in providing nitrogen to the plant. These root nodules
contain Rhizobium bacteria which are in a mutualistic relationship with the plant. The
lupin provides shelter and nutrients, the bacteria providing nitrates to the plant. The
presence of these Rhizobium bacteria imparts a reddish to the inside of the nodule.
A nodule forms when Rhizobium bacteria multiply around a root hair, these bacteria
secrete auxins which stimulate the root hair to grow around the bacteria. The bacteria
also secrete an enzyme which causes the development of a weakened region in the root hair,
they migrate through this region and eventually establish themselves in the cortex of the
root. The cortex cells become enlarged to form the root nodule. It is worthwhile noting
that bacteria will not invade the root if the soil is acidic - the presence of acidic
superphosphate will prevent formation of nodules in other legumes such as clover.
Soon after germination the stem develops a layer of fine hairs. These hairs trap humid
air lost from the epidermal cells, lowering the humidity gradient, and therefore reduce
further water losses by slowing the rate of evaporation.
In older plants a thick layer of bark forms, this provides protection from desiccation,
stem boring insects, as well as decreasing the damage caused by fires. After a fire the
plant is often able to re-grow its foliage from the undamaged cells within the stem.
As the plant grows the stem divides into many branches so that it forms a compact,
rounded shape. This means the interior regions of the bush are shaded, cooler and more
humid than the surrounding environment. These features provide a lower rate of water loss
for leaves on the inside of the bush.
The plant has compound leaves made of between 4 and 7 leaflets. They are covered with a
relatively thin cuticle (a waxy, waterproof layer) which is a little surprising for a
plant living in dry conditions. The leaves are covered in a layer of white epidermal hairs
which will help reflect heat away from the leaf and will also trap humid air lost by
transpiration from the stomata. This trapped layer of more humid air will slow
evaporation and therefore reduce further transpirational losses.
There are two surprising features about the leaves:
i. The upper side of the leaf has a greater density of stomata than the lower surface.
ii The lower side of the leaf has a much higher density of leaf hairs than the upper
Both of these are the opposite of what is found in most other plants. In most plants
the upper side of the leaf is hotter and therefore the stomata are normally found on the
cooler lower surface. In most plants the greatest density of leaf hairs is found on the
surface which has the greatest density of stomata.
The cause of this difference in leaf hair and stomatal distribution is caused by the
environment in which the lupin lives. The mostly bare ground gets very hot in the summer
sun and re-radiated heat from below so that the lower surface of the leaves may get much
hotter than the upper surface. The greater density of white leaf hairs on the lower
surface help to reflect some of this heat away. The greater density of stomata on the
upper surface means that their are less transpirational water losses than if they had been
concentrated on the lower hotter surface.
There is another adaptation which helps prevent excess water losses. The leaflets are
folded longitudinally down the length of the leaflet. This fold helps trap humid air lost
in transpiration by acting as a barrier it created a wind-shadow which means
the humid air tends to stay close to the lead allowing the formation of a humidity
gradient against the upper leaf surface. The folded leaflet also means that the light and
heat from the sun strike the leaf surface at a more acute angle and so the heat tends to
be reflected rather than absorbed by the leaf.
The folding of the leaf varies according to the amount of water loss. When the leaf is
losing a lot of water it is more tightly folded than when it is not losing excess water.
Leaves in the centre or sheltered side of the bush tend to be flatter, whereas exposed
leaves tend to be more folded to slow water losses.
Juvenile leaves have a higher surface area to volume ratio than mature leaves and so
would be expected to loose water at a greater rate than mature leaves. Because of this it
might be expected that the juvenile leaves will show greater leaf folding than mature
leaves under heat stress conditions.
Flowers occur in groups called inflorescences, the grouping of flowers and the fact
that the inflorescence is held vertically means that they are better able to attract bees
for pollination. The reproductive structures are well protected by the petals and are only
revealed when a bee lands of the flower.
The flower has two sets of stamens which mature at different times. The first set
mature before the stigma is ready to receive pollen this allows cross-pollination and
prevents self pollination. If cross-pollination does not occur then the second set of
stamens ripens so that self pollination will occur. This mechanism ensures that every
flower will produce seeds and so greatly enhance the survival and spread of the species
even in the inhospitable dry sandy shore ecosystem.
The seeds ripen in the pod (the fruit of the plant). At first the pod is green and
covered in a dense layer of white hairs which protect from desiccation and browsing
caterpillars. After the pod hardens it dries and on very hot days it splits and twists,
suddenly flicking the seeds over a considerable distance. The seeds are covered with a
very hard testa which provides protection and ensures the seeds remain dormant until the
next spring. The primary way the testa maintains dormancy is by preventing the entry of
water into the seed, without water the seed will not start to germinate.
If the seed did not have a way of maintaining dormancy then it could germinate in
summer after a heavy rain. Once germinated the seed would most likely die in the hot dry