In which plant is the given structure found?

In $Amorphophallus$ and $Colocasia$ $(Araceae)$, an extremely enlarged underground vertical stem meant for vegetative reproduction and storage is:

What is the function of the stem?

Which of the following adaptations is shown by $Opuntia$ (prickly pear)?

Plants of the tropics bear:

Give scientific reasons: Although ginger grows under the soil,it is a stem,not a root.