ingenuous [indзe'njuэs] (adj) artless; frank and candid; lacking in sophistication
純真な、無邪気な、率直な、飾り気のない

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He had never heard of the custom of sending flowers to one's lady-love, so he sent Genevieve fruit instead. There was utility in fruit. It was good to eat. Flowers never entered his mind, until, one day, he noticed a pale rose in her hair. It drew his gaze again and again. It was HER hair, therefore the presence of the flower interested him. Again, it interested him because SHE had chosen to put it there. For these reasons he was led to observe the rose more closely. He discovered that the effect in itself was beautiful, and it fascinated him. His ingenuous delight in it was a delight to her, and a new and mutual love-thrill was theirs--because of a flower. Straightway he became a lover of flowers. Also, he became an inventor in gallantry. He sent her a bunch of violets.
(The Game --by Jack London)