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Leaves and stems of plants, green vegetables or leafy greens, are widely consumed by humans. The protein contents are higher than in fruits, and they contain low amounts of sugar.

Some green vegetables produce secondary metabolites that have bitter or astringent properties and may produce toxic alkaloidal and other compounds such as hemoglutenens. Others produce intestinal enzyme inhibitors, such as lectins, which bind to mucosal surfaces and inhibit digestion, especially that of proteins. 

Plant secondary metabolism produces a large number of specialized compounds (~ 200.000) that do not aid in the growth and development of plants but are required for the plant to survive in its environment. Specialized compounds from secondary metabolism are essential for communicating with other organisms in mutualistic (e.g. attraction of beneficial organisms such as pollinators) or antagonistic interactions (e.g. deterrent against herbivores and pathogens). They further assist in coping with abiotic stress such as increased UV-radiation.

The broad functional spectrum of specialized metabolism is still not fully understood.

Well known specialized compounds include alkaloids, polyphenols including flavonoids, and terpenoids. Humans use quite a lot of these compounds, or the plants from which they originate, for medicinal and nutraceutical purposes.

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Franz Kafka

Now I can look at you in peace; I don't eat you any more. Apple