It's sitting on the right one of the two epsilonish plants.
… what Ms. Frizzle is doing?
Spoiler:
She's about to wrap Megan's rope in fabric and to stow it in her own bag. Megan approves. (Instead of burying the contaminated rope together with BraveBean, Ms. Frizzle wants to take it back to the Maybe someone can analyse it and figure out the nature of this danger.)