Tamara Bonnemaison concludes the exceptional seeds series with the following write-up:
Today the smallest seeds in the world–those of the orchids–are highlighted. I have chosen to feature this photo of the beautiful Aerides odorata, taken by Azhar Ismail (aka abiom.orchid3@Flickr), to serve as an example to learn about the minute wonders of the orchid seed. Thank you, Azhar! The accompanying illustration of orchid seeds is by J.G. Beer, published in 1863. This drawing is available via Wikimedia Commons.
Aerides odorata is an oft-admired epiphytic orchid species native to from India, Bangladesh, and Malaysia. It is one of the most profuse bloomers in its genus; the white and pink flowers have a delightful odour (hence the species name). The plants are prized by the floral industry. Like other orchids, Aerides odorata has extremely small seeds, measuring only 0.2mm in length. This is still four times larger than the smallest seed identified to-date, which belongs to another orchid, Anoectochilus imitans. To the naked eye, though, the seeds of both species would look like specks of dust. Royal Botanic Gardens, Kew has an informative article about orchid seeds–the photos of magnified orchid seeds are well worth a peek.
Why are orchid seeds so tiny? Orchids have a fascinating reproductive strategy; in order to germinate and grow, orchids in nature must be colonized by a specific mycorrhizal fungus. Having tiny seeds allows Aerides odorata and other orchids to produce millions of seeds at a very low energetic cost, making it more likely that some of those seeds will be deposited in close enough contact with an appropriate fungus. Most orchid seeds are small and light enough to float along wind currents, assisted by a one-cell thick testa that forms a balloon around the miniscule embryo. J.G. Beer’s illustrations shows this balloon-like testa around many of the seeds depicted. Most orchid seeds have extremely small food reserves in the embryo, and have foregone the endosperm altogether. They do not have the energy reserves required for germination and seedling growth, and must instead rely on their fungus to pass along carbon and minerals. The orchid seedlings are entirely dependent on this relationship until they are able to photosynthesize, and may continue to benefit from their mycorrhizal association during their entire lifespans. An article on the University of Sydney’s Fungal Biology website states that it is unclear what, if anything, the fungus receives in return.
The reproduction strategy employed by Aerides odorata and its kin has clearly worked well. The Orchidaceae is one of the two largest families of flowering plants (the other being Asteraceae), containing well over 25000 accepted species. There are also about 100000 hybrids and cultivars, which have been produced by gardeners and botanists enthusiastic enough to brave the strange world of orchid reproduction.