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Umbel - a flat-topped or rounded inflorescence in the shape of an umbrella with individual pedicels arising close together.
Umbilicus - a scar or depression on the summit of a fruit after the floral parts are shed.
Uncinate - Uncate - terminated by a sharply curved hook. Hence uncinatus - with hooked spines.
Undulate - wavy with alternately concave and convex regions. Hence undatus, undulatus - with wavy margins, undulated.
Under glass - grown in the greenhouse or, in the case of a cutting, with a jar or cloche placed over it.
Unequal - asymmetrical.
Ungui - claw-like. Hence unguispinus - with claw-like spines.
Uni - one, singular, as a prefix.
Uniseriate - (Latin: uniserialis) arranged in a single row, such as a single row of seeds in a pea pod. Having one line or series.
Unitegmic - with a single covering over a part e.g. ovule.