I understand that this is a very niche question. Please delete if it is encroaching on territory not covered by the fine folks at Ask Historians.
To avoid the winter doldrums I read about plants I can place in my garden for the following spring. As an added bonus I get to see lots of pretty floral ads on my browsers and mobile device (make the most of a bad situation, right?)
My latest object of interest is the marigold, in latin Tagetes. I've read that the common name of 'marigold' is supposedly derived from "Mary's Gold", 'Mary' of course being Mother Mary. Imagine my surprise when I discovered that Tagetes is native to the Americas and wasn't introduced to Europe until the 16th century! Once across the Atlantic the marigold seems to have grown in popularity in Europe and became naturalized in Africa and earned the common name "African Marigold". Strange story for a flower indigenous to the Americas.
My question is admittedly vague but could best be reduced to, What value did the marigold have to earn two trips across the Atlantic?
Was it an important flower to European visitors or colonial Americans? What other flowers have a similar story of worldwide travel? Why name the flower "Mary's Gold" if it wasn't even introduced to Europe (and the rest of the world) until the 16th century?
I'll take whatever you've got!
The name "Marigold" has been used in English (and Scottish, etc) to refer to several different plants. This includes several members of the genus Calendula. All of the members of the genus Calendula are native to Europe. One member of the genus, Calendula officinalis (sometimes called a pot marigold or a scotch marigold in modern gardening books), has large yellow ruffled flowers that look somewhat similiar to some members of the genus Tagetes. Here are some picture from Wikimedia Commons. Notice that the double-flowered variety looks very similar to the flowers of a Central American "marigold" species like Tagetes patula. In both genera, the common wild-type retains the typical flower shape of members of the Asteracea family.
Anyway, I think that's the story behind your question about why the new-world marigolds are called by the common names that they are. There are many other pairs of organisms, both plants and animals, that are called by a shared common name, but which the Old World and New World versions are not very closely related. The European versus American Robin springs to mind as an prominent example.