Friday, January 18, 2013

Why cheese plants are full of holes

Swiss cheese plants' leaves help them to avoid stress, according to a US scientist.

Their familiar hole-riddled leaves allow the plants to capture sunlight more regularly, his research suggests.

The counter-intuitive idea explains how such plants can survive in shady rainforests.

Commonly grown as house plants, they are found in the wild from southern Mexico to Colombia.

Many theories have been suggested for the unusual perforated leaves.

One is that the holes in the leaves allow the plants to resist hurricane winds, by letting the wind pass through. Another is that they allow better temperature regulation or water to run through the plants down to its roots. Some have suggested the holes somehow camouflage the plants, hiding them from herbivores.

But these ideas have rarely been scientifically tested.

Now research by graduate student Christopher Muir at the University of Indiana, in Bloomington, US, suggests the holes are an adaptation to the plants' rainforest habitat.

His findings are published in the journal The American Naturalist.

Leaves compared

The Swiss cheese plant Monstera deliciosa lives in the dark understorey of tropical rainforests. It relies on capturing unpredictable shafts of sunlight, known as "sunflecks", in order to photosynthesise for energy.

Mr Muir questioned whether the sunflecks could explain the unusual leaf shapes so he used mathematical models to compare leaves with and without holes.

He found that both leaf shapes benefit equally from the same amount of sunlight.

Although a leaf with holes will miss some sunlight, because it filters through the holes, solid leaves with the same surface area actually take up less space, so their access to sunlight is restricted.

Mr Muir's models revealed that a leaf with the same surface area, but riddled with holes, would contact sunlight more regularly because it takes up more space.

He suggests this regularity makes the altered leaf shape more reliable, causing the plant less stress giving it the best chance of survival.

But young Swiss cheese plants do not need holes in their leaves, according to Mr Muir.

Monstera deliciosa grows differently at different stages of its lifecycle; a relatively rare attribute among plants.

It is an epiphyte, or air plant, with aerial roots which attach to host trees, enabling it to climb. When young, the plant produces small leaves that are held close to the host tree's trunk.

The young plants are closer to the forest floor, where fewer flecks of sunlight reach. The poor quality of the light here means that holes do not benefit the plant, Mr Muir predicted.

Only as the plant matures does it grow taller, reaching parts of the understory with more flecks of sunlight.

Then the leaves become larger, develop holes and are held away from the trunk, where they have a better chance of capturing the sunlight necessary to survive.

"They parasitise the trunk and branches of their host plants to climb higher in the canopy where there is more sunlight," Mr Muir told BBC Nature.

Mr Muir's mathematical models suggest that leaf holes work best for plants that grow fast in shady areas, where flecks of sunlight are at a premium.

He suggests that biologists now test the idea by growing plants under different light conditions in the laboratory, simulating those found in the rainforest understorey.


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