Confusion with non-ideal current and voltage source models
This is probably from a textbook on IC design, or on circuit design with an emphasis on ICs.

They are assuming particular types of non-ideal voltage and current sources. The black bands in the resistive elements are likely intended to indicate the nonlinearity of these elements.

For the voltage source, they are considering a diode as the "typical" non-ideal voltage source you might want to model.

And for the voltage source they are considering a current mirror circuit as the "typical" non-ideal current source you might want to model.

Both of these are not actually power generators. They need power from the external circuit and that's why they don't work well near 0 V output (for the current source) or 0 current output (for the voltage source). 

Non-ideal power generators (like a chemical cell or electro-mechanical generator) would have very different characteristics.