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Leaves The canopy of the rain forest is usually closed, with plants retaining much of their leaf cover throughout the year. Fully deciduous trees are uncommon and may only account for a few percent of the tree species in the tropical rain forest. Rainforest leaves themselves share certain morphological features that are adaptations to the environmental pressures that they all have in common. They are usually leathery, a nice glossy green, and up to 90% may possess the distinctive ‘drip tip’ (Johns 1999). This is an extension of the end of the leaf, generally believed to have the effect of running the excessive moisture off the leaf. Leaf size in the best developed topical rain forest is relatively large, being generally described as ‘mesophyll’ (Whitmore 1999) or even larger. Of course, this is a generalisation, and leaf size varies depending on it’s position in the canopy.
In the Australian tropical rainforest, much of the splashes of colour are provided not by flowers, but by leaves. Leaves can be yellow, pink, red, purple or almost black. Unlike in temperate regions, many of these colours are not of leaves about to be dropped, but are often flushes of new growth. Many plants in the family Myrtaceae, such as the ‘Pendas’ and the Syzgium spp., have bright red and pink new leaves. Yellow leaves are usually leaves with undeveloped chlorophyll. However, red or pink leaves are coloured by a pigment called ‘anthocyanin’. This pigment reflects red light, which is normally used in photosynthesis. It is commonly found in fruits and flowers, and is responsible for the red colour in beetroot (Martin 1990). These bright colours may occur for several reasons. The presence of the pigment anthocyanin may be to protect the developing chlorophyll inside the new leaves from too much light, or high energy, damaging light such as ultraviolet. It has also been suggested that these colourful pigments may contain fungicides and other poisons (Breeden 1999). Another idea put forward involves the fact that many herbivores prefer soft, fresh growth to the older, tougher leaves and that the brighter colours of the fresh growth may serve as a warning that the new leaves are toxic or distasteful (Farrant 1999).
Some trees in the rainforest have the remarkable ability to flower and fruit on the branches and/or the trunk. These two conditions are called ‘ramiflory’ and ‘cauliflory’ respectively. Early explorers, and visitors today, have mistaken them for a parasite on the plant (Whitmore 1999). They can make for a spectacular show in the tropical rainforests. The example on the left below is from the tablelands, while the example on the right (Daintree Satin-ash) is from the Daintree lowlands
The roots of many tropical rain forest trees have many interesting variations. In many trees, the roots will spread as much horizontally as they do vertically. Erosion by running water may eventually reveal this complex network just underneath the ground. Many trees also develop large flanges above the ground that flank the tree on either side. These are called ‘buttress’ or ‘plank roots’ and give the tree the appearance of a rocket. Although some species are capable of developing them, and some are not, the development of buttress roots is a reflection of environmental pressures and not an indicator of species. Reasons suggested for such development include support and stability, allowance of air exchange in water-logged soils, and the capture of leaf litter. The example pictured on the right is from Marrdja boardwalk, Daintree. |
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