At this time of year I like to germinate and grow orchid seeds in Petri dishes with the help of symbiotic fungi. The “B1” fungus I mainly use is available from the Hardy Orchid Society. It has been identified as a Ceratobasidium strain from anastomosis group C (AG-C). It grows well on an agar medium containing nutrients from porridge oats and a tiny amount of marmite giving B vitamins etc.
I have taken some photos of germination using my simple microscope and a mobile phone camera – a bit fiddly, but it works!
Initially the orchid seeds have a small, brown, egg-shaped embryo inside a brown, protective sheath of dead cells called the testa. The structure is pretty much the same for all orchids. The embryo consists of about a hundred dormant cells that have their own intracellular nutrients, but no proper food store as the seeds of most other plants do. This is why they need to be infiltrated by, and draw nutrients from, specific fungi in the wild in order to grow.
The first change we see is that the embryos swell, accompanied probably by water imbibement. The cells wake up and start to divide. Growing larger, the embryo bursts out of the testa and at the same time several long rhizoids, or root hairs, grow out from it.
Some seeds can grow this far on their own, without any fungus, but no further. They run out of nutrients and die.
But provided a mutualism is established with the right strain of fungus then it grows steadily into a protocorm. This is a small, white, quite hard, little blob with one end that is usually more white, swollen, and rounded, from which the first shoot develops. The other end is thinner, more pointed and often brownish due to the fungus inside it. Many rhizoids develop, particularly from the pointed half, giving the protocorm a very hairy appearance.
These are the very earliest stages of development for the orchid plant. The protocorm becomes visible to the naked eye after about 3 to 6 weeks. This is the stage I have got to with the seeds I have germinated this autumn.
Photos: 1. Dry seeds of Bee orchid (Ophrys apifera) showing embryos within testas, 2. Southern marsh orchid seeds (Dactylorhiza praetermissa) with embryos starting to swell and fungal hyphae gathered around, 3. Same species with embryo near to bursting out and first rhizoids, 4. Protocorms of Green winged orchid (Orchis morio) developing, 5. Same, but more advanced with many rhizoids and distinct first shoot.
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