You might want to read the whole article. It's interesting.
> "In the painting, the black seeds indicate that the fruit has reached maturity," Wehner says.
There's also the possibility the painter simply decided to add black seeds because it would look more like a mature fruit.
I hope they would consider multiple credible sources corroborating the same thing. Is there a newspaper article from the time that describes watermelons as they are painted by Stanchi? Ads showing them as such? Is there a diary from a farmer that talks about how watermelons look and how they ripen, etc.?
Maybe there is, but the article sure isn't letting us know about it...
> "Museum paintings are an interesting method for studying old cultivars [varieties], and the one you indicated certainly shows the sort of watermelons that Europeans had to eat in the Middle Ages during their summer harvest season," Wehner says. "We have cultivars like that one in the painting available to us now from our germplasm collections [a sort of genetic sample library that includes many different varieties]."
> He notes that those samples, when grown today, have "large white areas, low sugar content, [and] frequent hollow heart." Hollow heart can cause a starring appearance somewhat similar to an unripe or underwatered melon.
So no, we don't need further corroboration from the 17th century, because we still have watermelons that look like that today.