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Orchid germination

In most species, small seeds, which are like dust, are dispersed by the wind and require nutrients provided by a mycorrhizal fungus to germinate.  Some members of Vanilloideae and Cypripedioideae have fleshy fruits that ferment in situ, releasing fragrant compounds (e.g., vanillin) to attract birds and mammals, which act as propellants.
These seeds are composed of an embryo of a few cells (between 100 and 200), covered by a hard testa.  The number of seeds can vary from 13,000 to 4,000,000 per capsule.  The weight range of an orchid seed varies from 0.3 to 14 mg and measure from 0.25 to 1.2 mm long, and is from 0.009 to 0.27 mm wide.

These seeds have no endosperm, and consist of a small embryo suspended within a membrane, typically transparent, although sometimes pigmented.  Forms can be very variable, such as elliptic, filiform, fusiform, round, globular or prominently winged.  All these features appear to maximize fertility and the effectiveness of wind dispersal of seeds of orchids.

The germination of these seeds occurs through a process that is different from the majority of angiosperms, because embryos of orchids are, anatomically and structurally, extremely small and simple.  The embryos of orchids germinate and grow to produce a mass of cells called protocorms.

These protocorms, with its rhizoids (root-like structures), may or may not immediately begin to photosynthesize. Nevertheless, for the protocorms to survive, develop and turn into shoot, they must first establish a symbiotic relationship with a fungus.
The role of the fungus is to provide the protocorms sugars (especially those who have no chlorophyll).  The fungus gets the sugar sections of the substrate (soil or other solid object to serve as a host organism to the plant) of the orchid, that is, the bark of a tree or the soil.  The protocorms, in turn, provides the fungus with certain vitamins and a habitat to live.

The fungus lives in the area of the protocorms and substrate.  Over time, the young scion will start producing its own nutrients, and symbiosis will no longer be necessary.



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