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Why do almost all plants in shade have a smaller stem structure and larger leaf than that same species grown in a well lit, sunny area?

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I have no idea of the biological mechanisms behind what I'm about to propose. Consider this a hypothesis.

If the purpose of the leave is to collect sunlight for photosynthesis, then the behavior you mention is predictable. I order of trying to maintain a healthy flux of light in dim conditions, the plant adapts by increasing the area of its leaves. Once back in a well-lit environment, the need for larger leaves fades and new leaves are thus smaller, but still catch as much light. Of course, there must be limits to the extent of leaf area variation that one plant can handle.

As for the stem structure, I can only guess it's because plants spend more energy on leaf building than stem maintenance when under such stresses.

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Also, plants that adapted to a "shady" ecological niche are competing less for access to direct sunlight, and therefore have less need to invest in height or the larger stem this requires. –  gremau Apr 13 '12 at 16:16
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The main factors that affect leaf shape are water and light needs. The plant needs a certain area of leaf to provide enough energy to live, so smaller leaves means more leaves(grasses), as opposed to one enormous leaf like a lilypad.

Generally, as light intensity increases water needs increase along with it. A thicker stem with a smaller leaf can support enough water flux to keep a grass blade hydrated in full sun, whereas a big tropical plant in high humidity has low water needs and can therefore use a thin stem to support a big leaf.

I would hypothesize that self-regulatory systems that affect leaf development as a function of available water and sunlight needs are responsible for the effect.

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