Why Are Sun Leaves Thicker Than Shade Leaves?

Light exposure can cause a plant to adapt the physical structure of its developing leaves.
Light exposure can cause a plant to adapt the physical structure of its developing leaves. (Image: fall yellow leaves and red dogwood leaves image by Jorge Moro from Fotolia.com)

When exploring nature, many observers notice that plant leaves adapt to their environment. As a plant grows, some leaves are exposed to the sun and other remain shaded. Typically, shaded leaves are located underneath sun leaves, and sun leaves are thicker. The different light conditions lead to physical changes in the structure of shade leaves and sun leaves.

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Shade Leaves vs. Sun Leaves

Generally speaking, shade leaves are the largest leaves of a plant. While shade leaves are bigger, sun leaves are thicker than their shaded counterparts. The differences in physical structure are caused by the cellular makeup of the leaves, according to the Oxford Journals website.

Palisade Cells

Palisade tissue makes up part of a leaf’s cellular structure, according to the Science and Plants for Schools (SAPS) website. Sun leaves are thicker than shaded leaves primarily because these leaves contain two to three layers of palisade cells. Shaded leaves generally only contain one layer of palisade cells.

Chloroplasts

The palisade tissue of the leaves differs because of the way shade and sun leaves use and capture light. According to the SAPS website, shade leaves and sun leaves have differing chloroplast structures. Chloroplasts help capture the sunlight and aid in photosynthesis, the process by which plants convert light into energy. In shade leaves, the chloroplasts are evenly distributed in the leaf, keeping it thin. In sun leaves, chloroplasts group together and position themselves in bunches where they can gather the most sunlight.

Mesophyll Surface Areas

Another structural element that changes the thickness of a leaf is the mesophyll surface area. The mesophyll surface area of a leaf is related to chloroplast production, according to the Oxford Journals website. The mesophyll is responsible for transporting carbon dioxide to the chloroplasts. As there are generally thicker groupings of chloroplasts in the sun leaves, the mesophyll structure is thicker to compensate.

Leaf Development

According to the SAPS website, the structural differences between sun leaves and shade leaves occur during leaf production and development. Mature, adult leaves are unable to change their structure if the lighting conditions change. This can explain why some people may observe sun leaves that appear brittle, damaged and thin, as observed on the Earth Expeditions website.

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