Characteristics of the Falling Speed of Japanese Orchid Seeds

Orchid seeds, which are produced in abundance, are particularly small and light. Some orchid species are anemochorous, i.e., bear seeds that are dispersed by wind. To characterize the seeds of Japanese orchids, we measured the size of seeds and embryos of 68 Japanese orchid species. Most orchid seeds had a length of 0.3 to 2 mm and a width of 0.07 to 0.2 mm. Embryo length and width were 0.1 to 0.3 mm and 0.04 to 0.2 mm, respectively. Twenty-seven orchid species produced 1000 to 350000 seeds per pod. Speeds of fall of 34 Japanese orchid species were examined in order to obtain insight into their seed dispersal ability. Falling speeds ranged from 4 to 30 cm/s. These results strongly suggest that seeds of Japanese orchids are also dispersed by wind and thus that most Japanese orchids are anemochorous species.


Introduction
The dispersal of seeds to a suitable habitat is a crucial event in the plant life cycle.Three major mechanisms spread seeds; i.e., hitchhiking on animals, floating in water, and moving in the wind (Fenner, 1985).Most anemochorous species produce particularly small and light seeds (Burrows, 1975).In some cases, seeds have wings that enable them to travel long distances (Burrows, 1973).Mathematical models have been developed to predict the dispersal distances of various anemochorous seeds (Cremer, 1977;Murren & Ellison, 1996;1998).
The orchid family is a morphologically diverse monocot.Because of their beautiful flowers, Orchidaceae plants are important not only for biologists but also in the marketplace (Sawa et al., 2006;Fukunaga et al., 2008;Ejima et al., 2011).The orchid produces dust seeds, with shapes that are adapted to the habitat of the orchid (Shimizu et al., 2012), and some orchids are considered to be anemochorous plants.However, orchid seeds have not been well characterized.Here, we characterized the seeds and embryos of Japanese orchids, and determined the speed of fall of the seeds.We discuss correlations between orchid habitat and properties of the seed, and comment on the evolution of this relationship.

Orchid Seeds
Orchid seeds were collected from their natural habitat in Japan.All seeds were dried using silica gel in a closed plastic box at least for two weeks at room temperature.

Seed Number
All of the dried seeds in one pod were weighed.Based on mass, the seeds were divided into ten groups, and the number of seeds was counted in one of these groups.The number of seeds per pod was calculated as being ten fold the counted number.

Seed Size
Seed size was examined using a stereomicroscope (Leica S8).Two-hundred seeds were evaluated per species.Mean value was shown in Table 1.

Speed of Fall of the Seeds
A triangular prism, 25 cm each side and 2 m in height, was prepared, one surface of which was constructed of paper and two of transparent vinyl.Top surface was made of paper.A hole, 5 mm in diameter, was made in the top surface.Seeds were dropped from the hole 20 times for each species, and measured the falling time.Falling speeds were calculated from the data of falling time and falling distance (25 cm).Mean value shown in Table 1.

Results and Discussion
We measured the seed size of 68 Japanese orchid species (Table 1).Orchid seeds are known to be small.In this analysis, we found that the seeds of Eria reptas had a length and width of 0.2 and 0.086 mm, respectively.This size is almost twice that of Zea mays (corn) or chestnut pollen grains.On the other hand, the seeds of the Lechanorchis group were quite long, with L. japonica producing seeds that were 4.6 mm in length and 0.15 mm in width.However, the embryo size of L. japonica was almost the same as that of other species (Table 1).
Seed number was also examined.Cymbidium goeringii produced 346900 seeds per pod (Table 1).In contrast, Amitostigma gracile, Ponerorchis kurokamiana, and Tulotis ussuriensis produced only 900 seeds per pod.We could not establish any correlation between seed size and seed number per pod.
To characterize seed dispersal ability, we measured the speed of fall of seeds from 34 Japanese orchids.The speed of fall of the seeds of Taraxacum japonicum, which is a well-known anemochorous plant, was 30.0 cm/s.Most of the seeds had slow speeds of fall (Table 1).Especially in the case of Habenaria dentata, Neofinetia falcate, and Bletilla striata, the speed was 29.8, 26.9, and 23.5 cm/s, respectively.Thus, most of the Japanese orchids could also be considered to be anemochorous plants.However, there was variation in the speed of fall of Japanese orchid seeds, which suggests variation in their method of dispersal.In particular, H. dentate and B. striata thrive in damp areas and on riverbanks, and their seeds can also be dispersed by water.The wind on the forest floor is weak, and orchids that grow in this habitat have to adapt to the weak-wind environment to disperse their seeds.Thus, the seeds of orchids living on the forest floor may have evolved to be long, as increased length reduces the speed of fall and increases the chance that the seed will be swept up by winds (Table 1; Shimizu et al., 2012).
Here we characterize the falling speed of Japanese orchid seeds.Most of the Japanese seeds can be considered as anemochorous plants.However, they seemed to evolve the seed shape to adapt their habitat environment, i.e. long and thin to reduce the seed falling speed.More detailed analysis of the seed shape will reveal how orchid adapt various environment in the world.

Table 1 .
Size and drop-speed of seeds on the native orchids in Japan