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It is easy to distinguish female plants of the species in the summer, fall and winter by the position of the flowers and fruit. Flowers and fruit are at the leaf axils on round leaf bittersweet and are only in terminal panicles on American bittersweet stems. Also, the fall fruit capsule color is yellow for round leaf bittersweet and orange for American bittersweet. It is more difficult to distinguish male plants because they do not set fruit. Flower position is still indicative. During the spring, leaf unrolling is also indicative. Conduplicate (folded in half lengthwise with the upper side inward) leaves are round leaf bittersweet and involute (inward curling) leaves are American bittersweet. Other potential characteristics such as leaf shape (round leaf bittersweet has rounder leaves) and fruit number per cluster are highly variable and indefinite.

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American bittersweet fruit capsules are orange
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Round leaf bittersweet fruit capsules are yellow
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American bittersweet fruits are at the terminal 
stem ends
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Round leaf bittersweet fruits are along the stem
at leaf axils
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Round leaf bittersweet leaf shape is highly variable and unreliable for identification