What is the function or reddish abaxial leaf surface?

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What might be the function (if any) of the reddish color of the abaxial leaf surface of some Philodendrons? I have acuired a juvenile specimen of a houseplant (presumably Philodendron sagittifolium or a Holland hybrid) which has leaves colored reddish on the abaxial surface. The undreside of some leaves are more reddish than those of others, but the adaxial surface is always green. I have read that juvenile specimens of Philodendron melinonii and maybe other species has a reddish underside too. Why is it good for the plant to have a reddish leaf surface on the underside? I can imagine that a reddish color could provide protection of leaf tissues from excess sunshine, but I can't see any meaning of it on the underside. 

Ferenc

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