Which is the world's smallest flower?

Answer 1

Wolffia
A bouquet of one dozen plants in full bloom will easily fit on the head of a pin.

There are approximately 250,000 species of described flowering plants in the world, and they range in size from diminutive alpine daisies only a few inches tall to massive eucalyptus trees in Australia over 300 feet tall. But the undisputed world's smallest flowering plants belong to the genus Wolffia, minute rootless plants of the duckweed family (Lemnaceae) that float at the surface of quiet streams and ponds. Each plant is shaped like a microscopic green football with a flat or rounded top depending on the species. They reproduce exponentially by budding, and during warm summer months they literally cover the water surface like masses of granular, green Malto-Meal. Two of the smallest species are W. angusta, an Australian species recently described in 1980 by Dr. Landolt, and the worldwide tropical species W. globosa. The entire plant body of these two species is less than one mm long (less than 1/25th of an inch) and it is difficult to say which is the smaller of the two, but perhaps W. globosa may be slightly smaller. An average individual plant is 0.6 mm long (1/42 of an inch) and 0.3 mm wide (1/85th of an inch). It weighs about 150 micrograms (1/190,000 of an ounce), or the approximate weight of two ordinary grains of table salt. One plant is 165,000 times shorter than the tallest Australian eucalyptus tree and 7 trillion times lighter than the most massive giant sequoia tree. It is small enough to slip through the eye of an ordinary sewing needle, and at least 5,000 plants could be packed into a thimble. Each plant produces a microscopic flower inside a small cavity that develops on the upper side of the plant body. The minute flower consists of a single pistil and stamen. Since the stigma is generally receptive before the anther is mature, a condition known as protogyny, the flower typically requires cross pollination from a different wolffia plant with a mature anther that is ready to shed its pollen. A bouquet of one dozen plants in full bloom will easily fit on the head of a pin. After pollination the ovary develops into a tiny one-seeded fruit called a utricle, which also holds the record for the world's smallest fruit.

Answer 2

Scarlet Pimpernel.

Answer 3

Wolffia

Answer 4

Wolffia seems to be it. Check this site:
http://waynesword.palomar.edu/plmar96.htm