Flask tightly sealed
 

After 2 weeks, I observed that my homemade sowing medium is free from contamination. thus I plan to sow my orchid seeds. My question is, right aafater I sow the seeds in the medium, should I seal the flask tightly? What happened to the cultured inside the flask if there is no air/oxygen? Anybody can advise me what to do.

The plants don't use enough C02 during and after germination to need any extra air than what's in the flask, so sow away and seal it up tight!

When it's time to replate the seeds into another flask so they can grow bigger, you want to use a 'vented' flask, which is just a flask with a hole in the lid, with the hole covered with cotton, Tyvek, micropore tape, or something else that lets air in while filter mold spores out. That should give the slightly larger plants enough C02 to grow while in the flask.