Leaves enable the plant to covert sunlight into energy through photosynthesis. The surface area of the leaf is very important when it comes to their ability to absorb sunlight, which is mainly absorbed by a thin layer of cells just below the surface of the leaf.
In shady areas having a larger leaf can give a plant an advantage over its smaller leaved neighbours, as bigger leaves can absorb more light. In the lower layers of the rainforest there is little light because of the shadow caused by the canopy. This means the leaves need to be bigger to absorb more light to help them survive.
Surface area affects water loss, so large leaves will loose water more quickly through their stomata cells located on the underside of the leaf. Depending on the location, leaves in a shady place will often loose less water than those in sunlight; due to possibly lower temperatures, less wind and that shaded leaves will also likely be more sheltered. This means shaded leaves can often get away with being bigger too.
To grow big leaves, a tree needs plenty of nutrients. Wet soil encourages
decomposition of plant and animal matter in the soil, which in turn means that the soil of rainy places often contains a lot of nutrients. This rich soil allows the trees that grow in it to more easily produce big leaves. The soil in a dry location would have slower decomposition, so nutrients may be harder to come by.
Moisture and heat can affect the size of plant leaves. Some plants can reduce water loss through the stomata by fixing carbon at night. Cacti have modified their leaves to become thin spines which do not lose water through transpiration.
Leaves also collect rainfall and direct it to the tree's root system. The environment of a rainy location encourages the trees there to grow big leaves.
The amazon waterlily (Victoria amazonica) has giant leaves that grow up to 3 metres in diameter. They spread across the water like rafts and can support a child weighing 30 kg / 200 pounds on a single leaf.
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