Question of the Day – What are the world’s smallest flowering plant, fruit, and seed?
To be an angiosperm, a plant must produce a fruit of some kind and have enclosed seeds (angio = vessel, and sperm = seed, so seeds in a vessel). It will have a pistil and/or stamen, and if fertilized, the embryo becomes a seed and a fruit formed from the ovary (and perhaps other parts).
There is no size requirement to be an angiosperm, it just has to be big enough to carry the requisite anatomical features. Flowering plants run the gamut of sizes, from huge trees to small Australian violets (Viola hederacea) at only 1.5 in. (3.8 cm). But even this tiny violet, with its 0.25 in. (6 mm) flowers is huge compared to the smallest of the flowering plants.
Imagine a thimble filled with plants. How many plants? How about 5000! Not seeds mind you, but fully mature plants. This is easy for the watermeal plant (Wolffia globosa), the world’s smallest flowering plant. When you pick up a single plant (if you can), you can hardly see it on your finger. The entire plant is only 0.6-0.8 mm long, about the same size as a grain of salt, and weighs only 150 micrograms (0.00015 grams).
It takes a determined plant to fit itself into such a small volume. Decisions must be made about what is necessary and what can be lost. In evolutionary terms, this is called reduction. W. globosa is a greatly reduced plant. It has no roots, no leaves, no petals, and no stem to speak, although developmental studies show that it's mass is part stem, part leaf.
W. globosa is also one of the fastest growing plants in the world. It can double its biomass in just 30 hours. We think of bamboo as a fast growing plant, and it is, but doubling time for a bamboo plant can is measured in days or weeks, not in hours.
The water hyacinth (Eichhornia crassipes) was supposedly the fastest growing angiosperm in the world, with a biomass doubling time of six days under the best growing conditions. I think this was probably before they started looking at watermeal in more depth. It could be easy to overlook.
The fast reproduction and growing time for W. globosa means that it can completely cover a pond in a matter of days. This reduces the amount of sunlight for underwater plants, and crowds out the photosynthetic phytoplankton. Dissolved oxygen will become depleted. This could lead to a fish kill that would decimate the entire pond. Watermeal is so small that it is easily transferred to other bodies of water on the feathers and feet of ducks, so it is invasive. “Reduced” apparently doesn’t apply to survival capability.
Wolffia gets away with being leafless because it has chloroplasts in the cells of its body. I don’t really know what to call the body of watermeal. It isn’t a stem, since a stem connects different structures of a plant to one another. In the case of W. globosa, there is nothing to connect to anything else.
There are other plants that don’t show a stem above ground, but they have a connection for leaves to the root, and these are called acaulescent (a = without, and caulis = stem) plants. For example, many succulents have thick leaves that come straight out of the ground.
W. globosa does have a flower, but you wouldn’t recognize it. The flower is held in a small cavity on the top of the football shaped plant (see picture). The flower is also reduced, having only one pistil and one stamen. Pistil is the name for the complete female structure, including the stigma, the style, and the ovary. The stamen is the name given to the complete male structure, including the filament and the pollen-producing anther.
These are the only parts of a flower that are necessary; many flowers just have one or the other (male or female flowers). Plants that have both types of flowers on a single individual are called monoecious, while plants with just one type or the other are dioecious. So even though the flower of the watermeal has no petals and is only 0.2 mm in diameter, it is functionally more complete than flowers that are thousands of times its size.
W. globosa is an exception in that it is an angiosperm that most often reproduces through asexual means. Watermeal usually buds, much like yeast or coral polyps. The bud grows from the end of the mature watermeal and can be as large as the parent plant. This is why W. globosa is fast growing; by the time the bud separates, it is a mature plant.
In the rare cases that it is pollinated, just one seed is produced. You would think that it would be the world’s smallest seed, but it isn’t - not by a long shot. The W. globosa seed is 0.3 mm, between half and ¾ the size of the fruit, but some orchids have much smaller seeds.
The coral root orchid (Corallorhiza maculata) has seeds that measure just 0.085 mm each – there are bacteria larger than that! There are many similarities between the coral root and W. globosa. Coral root doesn’t have leaves or roots to speak of, just like watermeal. It is parasitic and gathers its nutrition from the soil fungi. The main difference is that while wolffia produces only one seed, the orchid has thousands, easily dispersed by the wind, since it takes 375,000 of the to equal one ounce (28 g).
Could this speck of a watermeal plant impact humankind? You betcha. Soybeans are supposed to be the world’s superfood, being about 40% protein, but wolffia has the same amount of dietary protein as soybeans, with more of the essential amino acids that humans must gain from our food. And watermeal produces protein 50x faster than soybeans.
W. globosa is already used a vegetable in southeast Asia, but with its high protein and carbohydrate concentrations, small size, easy growing conditions and rapid maturation reproduction, it may be much more. There are scientists who are proposing that watermeal form the basis of the astronaut on trips to Mars and beyond.
Xie, W., Huang, Q., Li, G., Rensing, C., & Zhu, Y. (2013).CADMIUM ACCUMULATION IN THE ROOTLESS MACROPHYTE AND ITS POTENTIAL FOR PHYTOREMEDIATIONInternational Journal of Phytoremediation, 15 (4), 385-397 DOI: 10.1080/15226514.2012.702809
Zhang, X., Uroic, M., Xie, W., Zhu, Y., Chen, B., McGrath, S., Feldmann, J., & Zhao, F. (2012). Phytochelatins play a key role in arsenic accumulation and tolerance in the aquatic macrophyte Wolffia globosa Environmental Pollution, 165, 18-24 DOI: 10.1016/j.envpol.2012.02.009