Ponderosa lemon
The ponderosa lemon originated in roughly 1887, and is believed to come from a chance seedling grown in Hagerstown, Maryland. It was later named and introduced to the nursery trade in 1900. Though often referred to as a cross between the lemon and citron, a recent genomic analysis showed it to solely contain pomelo ("Citrus maxima") and citron ("Citrus medica") DNA, perhaps being an F2 hybrid. Thus they cannot derive from true lemons, which have a mandarin orange component.
Citron
Today the citron is used for the fragrance or zest of its flavedo, but the most important part is still the inner rind (known as pith or "albedo"), which is a fairly important article in international trade and is widely employed in the food industry as succade, as it is known when it is candied in sugar.