FRUITS AND SEEDS

cross-section of an orchid capsule, showing 3 or 6 longitudinal slits
The orchid ovary is always inferior (located behind the flower), three-carpelate and one or three-partitioned, with parietal placentation (but axile in the Apostasioideae).
If pollination was successful, the sepals and petals fade and wilt but they remain attached to the ovary. The epigynous ovary typically develops into a capsule that is dehiscent by 3 or 6 longitudinal slits, while remaining closed at both ends. The ripening of a capsule can take 2–18 months. The microscopic seeds are very numerous (over a million per capsule in most species). They blow off after ripening like dust particles or spores, barely visible to the human eye. Since they lack endosperm, they must enter symbiotic relationship with mycorrhizal fungi to germinate. These fungi provide the necessary nutrients to the seeds.

An orchid seed capsule
All orchid species are reliant upon mycorrhizal associations with various basidiomyceteous fungi to complete their lifecycle. Although all orchids are mycoheterotrophic during germination, some achlorophyllous (lacking chlorophyll) species are entirely dependent upon these fungi for nutrients. In general, orchid mycorrhizal fungi decompose organic matter and subsequently translocate the obtained nutrients via their hyphae to the orchid. Because most orchid seeds are extremely tiny with no food reserves (endosperm lacking), they will not germinate without such a symbiont to supply nutrients in the wild. Some fungi continue to live in the roots of the adult orchid. This enables an orchid such as Neottia nidus-avis to function without chlorophyll. The chance for a seed to meet a fitting fungus is very small. Of all the seeds released, only a minute fraction grow into new orchids. This process can take years; in some cases up to fifteen years.
Horticultural techniques have been devised for germinating seeds on a nutrient-containing gel, eliminating the requirement of the fungus for germination, and greatly aiding the propagation of rare and endangered species.

SOURCE:WIKIPEDIA