Learn About Orchids and Orchid Care

Learn About Orchids and Orchid Care

Tuesday, March 15, 2005

Fruits and Seeds

The orchid ovary is always inferior (located behind the flower), three-carpelate and 1 or 3-celled, with parietal placentation (but axile in the Apostasioideae).

If pollination was successful, the sepals and petals decolorize 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 from 2 to 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. These provide the necessary nutrients to the seeds.

All species rely upon mycorrhizal associations with various fungi, mostly genus Rhizoctonia (class Basidiomyetes), for at least part of their life cycle. Some achlorophyllous (lacking chlorophyll) species are entirely dependent upon these fungi. The relationship between fungi and the plant is often called symbiotic, but it is not at all clear what, if anything, the fungi derive from the relationship. It has been referred to by some as "mycotrophic", meaning that the plant is parasitic upon the fungus. At the very least, the fungi decompose surrounding matter, freeing up water-soluble nutrients. 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 live on 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 grows into a new orchid. 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. Germination can be extremely slow.


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