Answer: Milky sap is usually considered to be an adaptation for protection, probably against insect attack. At the extreme, plants such as rubber trees have a milky sap called latex that is the raw material for rubber. Many rainforest plants have milky sap, and this is consistent with the insect-deterrent theory, considering how many insects inhabit the rainforest, and the fact that many rainforest trees have relatively thin, easily penetrated bark.
Other familiar plants with milky sap include poinsettia, milkweed, ficus, and even dandelions! Though I can't say this for sure, I suspect that young lettuce plants begin to develop the milky sap early, but it simply doesn't show because it doesn't exude from the smaller veins the way it does from a large stem.
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