This is a maple tree seedling...if you didn't catch it already, it's the little sprig
of leaves in the white circle. It will eventually grow into a tree, probably a vinemaple
since they're the native variety.
The greenhouses were designed to will tiny seeds to life, growing
them into seedlings large enough to survive outside. They start out in trays like
these, which are thinned out into larger trays, thinned into little pots, and up
and up until they're ready for the outside world.