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 the longest and shortest known seeds in the family being 120-fold. The ' widest ' 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 µg to 24 µg (a 78-fold difference). Recorded numbers of seeds per fruit are as high as 4 000 000 and as low as 20-50 (80 000-200 000-fold difference). Testae are usually transparent, with outer cell walls that may be smooth or reticulated. Ultrasonic treatments enhance germination, which suggests that the testae can be restrictive. Embryos are even smaller : their volume is substantially smaller than that of the testa. As a result, orchid seeds have large 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 spaces enable the seeds to float on water for prolonged periods. This facilitates distribution through tree effluates and\or small run-off rivulets that may follow rains. Due to their size and characteristics, orchid seeds may also be transported in and on land animals and birds (in fur, feathers or hair, mud on feet, and perhaps also following ingestion).