Tansley review No. 110 - Numerical and physical properties of orchid seedsand their biological implications

Orchid seeds are very small, extremely light and produced in great numbers. Most range in length from c. 0.05 to 6.0 mm, with the difference between t he longest and shortest known seeds in the family being 120-fold. The 'wide st' seed at 0.9 mm is 90-fold wider than the 'thinnest' one, which measures 0.01 mm (because orchid seeds are tubular or balloon-like,'wide' and 'thin ' actually refer to diameter). Known seed weights extend from 0.31 mu g to 24 mu g (a 78-fold difference). Recorded numbers of seeds per fruit are as high as 4000 000 and as low as 20-50 (80000-200000-fold difference). Testae are usually transparent, with outer cell walls that may be smooth or retic ulated. Ultrasonic treatments enhance germination, which suggests that the testae can be restrictive. Embryos are even smaller: their volume is substa ntially smaller than that of the testa. As a result, orchid seeds have larg e internal air spaces that render them balloon-like. They can float in the air for long periods, a property that facilitates long-distance dispersal. The difficult-to-wet outer surfaces of the testa and large internal air spa ces enable the seeds to float on water for prolonged periods. This facilita tes distribution through tree effluates and/or small run-off rivulets that may follow rains. Due to their size and characteristics, orchid seeds may a lso be transported in and on land animals and birds (in fur, feathers or ha ir, mud on feet, and perhaps also following ingestion).