Pollination of the World’s Smallest and Largest Plants
By: H. Wayne Shew, Ph.D.
NAB Certified Pollen Counter
Flower-producing plants (angiosperms) come in a variety of sizes and shapes but they all share the need to have pollen grains (the male gametophytes) transferred to female gametophytes in order to effect fertilization of egg cells and to bring about formation of seeds. The pollinators that are needed to ensure that pollen gets transferred from one flower to another also varies, and include a variety of animals as well as water and wind. In this blog I want to talk about the smallest and largest flowers and the manner in which they are pollinated. In a subsequent blog I’ll address the smallest and largest pollinators of flowering plants.
The smallest flower known is produced by plants in the genus Wolffia. There are five known species and probably the smallest is W. globosa, a plant which averages 0.6 mm in length and 0.3 mm in width, and which weighs approximately 150 µg. Plants of this species produce a microscopic flower that possesses one carpel (pistil) and one stamen. The flowers require cross-pollination since the stigma is not receptive at the same time that the anther matures inside the flower and releases its pollen. Thus, pollen must be transferred from one flower to another, a process brought about through the action of water; Wolffia is a plant that grows in water as is true for other members of the duckweed family. A dozen of these W. globosa plants in full bloom would fit on the head of a pin. There are species of bacteria (which are prokaryotic organisms—their cells lack nuclei) that are larger than this angiosperm (which are eukaryotic organisms—their cells have nuclei). Wolffia reproduces asexually as well as sexually. In fact, its principal means of growth is through asexual reproduction. A plant can bud to produce a smaller daughter plant every 30-36 hours. Under ideal conditions, a single Wolffia plant could fill a volume equal to the size of the earth in approximately four months through asexual reproduction. An interesting aside is that these plants are edible, with a molecular makeup of about 40% protein and an amino acid content similar to that of soybeans. These plants are called “khai-nam” in Thailand and are eaten by people there and in other countries of Asia.
The largest flower is produced by plants in the genus Rafflesia. This genus is found in rainforests of southeastern Asia, mainly in the countries of Malasia, Indonesia, Thailand, and the Philippines. The genus not only produces the largest flower, but also the heaviest (weighs upwards of 20 pounds, and the rarest. The Rafflesia plant lacks stems or roots and is parasitic on the host plant Tetrastigma, a vine which grows only in undisturbed rainforests. The flowers of most species are unisexual, but they all produce an odor like rotting flesh and are locally called corpse flowers. The odor attracts insects such as flies which are responsible for transporting pollen from male flowers to female flowers. Obviously, the size of the flower is not always related to the size of the pollinator, and neither is the size of the flower directly related to the size of the pollen grains it produces.
We’ll look at the smallest and largest pollinators known in a subsequent blog.