Why is pith of Cucurbits hollow when mature?
Pith autolysis is a phenomenon due to which in some dicotyledonous plants like cucurbits, the pith gets hollowed.

Background: We all know that plants get their Carbon from atmosphere in the form of Carbon-dioxide, and fixes it by photosynthesis. During rapid growth Carbon requirement may not be in pace with rate of carbon fixed by photosynthesis alone, So some plants adopted pith autolysis.

This is useful for a  plant when it is in scarcity of carbon, Scarcity here implies that the plant is not able to get enough carbon for its growth with just photosyntgesis. So the plant autolyses it's pith to get required carbon to maintain its growth.Thereafter this carbon resulting from autolysis ,is transported to growing regions of plant.

Source:https://doi.org/10.1006/anbo.1995.1063