Given scientific fact as input, generate the question from this fact such that it can be answered from the input.

The vascular structure functions as a framework that reinforces the shape of the leaf.
What functions as a framework that reinforces the shape of the leaf?

Protists are thought to be the oldest eukaryotes.
What is thought to be the oldest eukaryotes?

Binary transition-metal compounds, such as the oxides and sulfides, are usually written with idealized stoichiometries.

Binary transition-metal compounds, such as the oxides and sulfides, are usually written with idealized what?