On Country

Show PowerPoint – All about leaves (TOOLBOX)

Flowering plants have been evolving for about 150 million years; time enough to adapt to the many different environmental conditions on Earth during that time.

Morphological adaptations in a plant

• Stems instead of leaves:

• Wattles have adapted to dry regions by a dramatic reduction in leaf surface. In Acacia rigens, leaves are needle-like, thereby reducing water loss and contact with heat radiation.

• In many species, such as A. saligna, leaves are dramatically reduced. Instead, the flattened and widened leaf stems (called phyllodes) develop and function as ‘leaves.’ Stems have thickened cell walls and contain few stomata compared with a leaf. Thus, transpiration is reduced, and water is conserved.

• The witchetty bush (A. kempeana), is well-adapted to hot conditions. The surface of the phyllodes is leathery, thickened with cellulose, which reduces evaporation. In addition, the phyllodes are often held in a drooping position, which reduces the direct infra-red rays from the Sun.

• Sheoaks (Allocasuarina spp.) have a similar reduction of their leaves. The ‘leaves’ are actually leaf stems, bundled together. The true leaves are reduced to a tiny whorl around the beginning of each bundle of stems.

• Leaf texture:

• Leathery leaves, greyish green in colour, particularly quandong trees (Santalam acuminatum, have the outer layer thickened with cellulose.

• They may also have oil glands, such as Eucalyptus spp. An oily surface retains moisture.

• Thick leaves denote water storage in cell vacuoles of both leaf and leaf stem. (eg. cactus, Carpobrotus (kolbogo)

• Leaf colour:

• ‘Grey’ leaves have a surface of very tiny white ‘hair-like’ protuberances from the outer layer of the cell wall that act to reflect the Sun’s heat radiation.

• ‘Hairs’ on leaves (and some flowers) divert the passing of dry air over the surface, reducing evaporation (eg. hairy petals of Mulla mulla, of leaf and fruit of bush tomato, Solanum sp.)

• In addition, they create a microclimate of humidity trapped at the leaf surface.

• Leaf surface:

• Sclerophylly (or hard leaves) is the term used to describe leaves that are tough and rigid and usually have a very thick outside surface (cuticle).

• Leaf shape or no leaves at all!

• A broad leaf can be divided into sections, as in the Desert Kurrajong (Brachychiton gregorii). This formation breaks up the wide surface of a large leaf, effectively reducing the surface for transpiration.

• The Desert Kurrajong drops all its leaves in conditions of very high temperatures. As dry air passes over the leaves, transpiration may be greater than the tree’s ability to draw in water stored in the roots.

• Roots:

• The desert kurrajong (Brachychiton gregorii) and particularly those of mallee eucalypts, store water in enlarged vacuoles (spaces) in the root cells.

• This was well-known to Aboriginal people who dug up the roots, cut them open and ‘water flows freely from them.’(1)