Leaves function to capture carbon dioxide and sunlight to produce carbohydrates (sugar, starch and cellulose [or wood]), while minimizing water loss. Protea leaves possess lots of woody (lignified) tissue, occasionally with special supporting tissue. These features of sclerophylly prevent leaves from collapsing when water is scarce (the drought hypothesis). However, sclerophyllous leaves are expensive to produce and restrict water loss, thus hindering photosynthesis by the consequent reduction in carbon dioxide uptake (they are thus a high cost, slow profit, water-saving leaf). Because they are drought resistant, sclerophyllous leaves can continue photosynthesizing long after ordinary (orthophyllous) leaves would have wilted. They are also longer-lived than orthophyllous leaves, compensating for their high cost. The leathery, woody nature of sclerophyllous leaves also appears to be a structural feature of plants growing on nutrient-poor soils, especially where nitrogen and phosphorous are scarce. Because nitrogen and phosphorous are needed for protein synthesis, the excess carbon from photosynthesis ends up as non-protein and non-chlorophyll compounds, such as woody fibres and tannins (the nutrient hypothesis).
Sclerophyllous leaves have a high ratio of carbon to nitrogen which make them indigestible to most insect larvae (since insects require nitrogen for proteins). Similarly, very few mammals will eat protea leaves, as the high carbon to nitrogen ratio is compounded by low levels of essential trace elements -leading to deficiency diseases in herbivores - and chemicals (tannins) - which impair digestive enzymes in the gut making digestion inefficient. Few insects eat protea leaves. Those that do eat them when they are young and succulent. Mammals which try to live exclusively off protea leaves would soon die of starvation, and if they lived long enough would end up with serious deficiency diseases (such as "rickets").
In the Cape Floral Kingdom four major types of protea leaves can be recognized:
Leaf tips in proteas differ considerably. In some species leaves end in a little sharp red or black point, called a mucron. These may be very sharp (as in Pr odorata), and probably serve to discourage browsers. However, in some cases mucrons may be fleshy in young leaves and probably serve the same function as glandular tips.
Glandular leaf tips are most conspicuous in the Pincushions (Leucospermum), which may have up to 20 glands per leaf, but occur in most genera. They secrete a sugary solution and are thus extrafloral nectaries. Their exact function is unknown, but they are visited extensively by wasps, ants and flies. It is possible that they might excrete excess water, cool leaves by secreting moisture, or contain substances which prevent herbivory. However, wasps and ants parasitize and prey upon other leaf-eating insects, and their presence may perhaps reduce herbivore numbers. In some species which produce little nectar, they might help attract insects to flower-heads.
The thickened leaf margins of many Protea and Paranomus species do not appear to secrete nectar. They may perhaps serve as a strengthening and cooling devices for the large leaves. However, their function is not known.
Protea leaves may be hairless or covered with hairs, wax or denticles. Hairs may be long or short, straight or (in Leucospermum only) curly. Most silver and grey leaves have dense layers of hairs, which reflect bright sunlight and may thus prevent overheating. Hairs also prevent wind from disturbing the air at the leaf surface, which allows efficient carbon dioxide uptake while reducing water loss. Hairs might hinder insect herbivory, by clogging up mouthparts or increasing chewing time. In some bird-pollinated Mimetes species hairs contribute to the visibility of the flower-heads.
Some proteas have fine denticles on their leaves, making them sandpapery to the touch (scabrous). These are modified hairs, which probably serve both a supportive and anti-herbivore function. In Protea denticulata these denticles are so rough that the bushes with their interlocking branches resemble "barbed wire entanglements".
Some blue-grey (glaucous) leaves are not covered by hairs, but by fine waxy scales or `bloom'. Glaucous leaves are another solution to reducing the amount of light reaching leaves. By reflecting light, the silver scales prevent overheating and the wax aids in keeping the leaf surfaces impermeable to water. Wax may also hinder herbivores.
Young leaves present a problem for proteas growing in nutrient-poor soils. New growth is soft, since new leaves must expand before the strengthening woody components can be laid down. They are also full of protein which is needed for rapid growth. Consequently, they are choice items on any herbivore's menu. They are also soft and therefore cannot resist desiccation like older leaves. In order to overcome these problems young leaves must grow quickly and are usually covered by dense hairs and waxy blooms. They are often a different colour to mature leaves, usually red or yellow. The reason for this is not known, but may pertain to reducing heat load (red leaves reflect more heat) and thus water loss. Also, many insects cannot readily see red light, and although brightly coloured to humans, red leaves may be relatively inconspicuous to insect herbivores.
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