Commelina is a genus of approximately 170 species commonly called dayflowers due to the short lives of their flowers. They are less often known as widow's tears. It is by far the largest genus of its family, Commelinaceae. The Swedish taxonomist Carl Linnaeus of the 18th century named the genus after the two Dutch botanists Jan Commelijn and his nephew and Kaspar, each representing one of the showy petals of Commelina communis.
The dayflowers are herbs that may be either perennial or annual. They are characterised by their zygomorphic flowers and by the involucral bracts called spathes that surround the flower stalks. These spathes are often filled with a mucilaginous liquid. Each spathe houses either 1 or 2 scorpioid cymes, with the upper cyme being either vestigial or bearing 1 to several typically male flowers, and the lower cyme bearing several flowers. All members of the genus have alternate leaves.
The Asiatic dayflower (Commelina communis) is probably the best known species in the West. It is a common weed in parts of Europe and throughout eastern North America. Several species, such as Commelina benghalensis, are eaten as a leaf vegetable in Southeast Asia and Africa.
The inflorescences are terminal, meaning the stem terminantes with an inflorescence, and often leaf opposed, meaning it emerges at the node with a leaf of a new axiallary stem. The inflorescence is composed of 1 or 2 cincinni, also called scorpioid cymes, which are monochasiums (i.e. cymes with a single branched main false axis) in which the lateral branches arise alternately on opposite sides of the false axis. The distal cincinnus may either be vestigial or contain one to several flowers that are typically male. The proximal cyme is always present and is several flowered. The cincinni are enclosed in a folded spathe, a modified leaf, which is often filled with a mucilaginous liquid. The spathe may either have completely distinct margins or they may be fused to varying degrees at the basal end.
The flowers are borne on pedicels and are strongly zygomorphic, meaning there is only a single plane of symmetry. Bracteoles occasionally subtend the pedicels, but they are usually absent. The flowers are either bisexual or male. There are 3 unequal sepals, which may either be free or the 2 lateral ones may be fused. The petals are free and unequal with the two upper ones being larger and clawed while the lower petal is typically reduced and often differs in colour from the other two. Flower colour is most typically blue, but lilac, lavender, yellow, peach, apricot and white also occur. There are 3 stamens and 2 to 3 staminodes, or infertile stamens, all of which have free, glabrous filaments. The staminodes occur posteriorly and have antherodes with 4 to 6 lobes. The stamens are anterior and are longer than the staminodes. The medial stamen differs in size and form from the lateral 2, and when a central staminode is present it also differs from the other staminodes. The ovaries are bi- or trilocular and one to two ovules is present per locule.
The fruit is a capsule that is typically bi- or trilocular, but in rare cases may be unilocular, and it is bi- or trivalved. The locules may contain between 0 and 2 seeds. The seeds are uniserriate (i.e. arranged in a single row), have a linear hilum and a lateral embryotega.