Anyway, as I was leaving the gallery, a painting of a fruit caught my eye. The fruit pictured was exactly the fruit that I had written about a few days ago. I now know that this fruit is called Soursop (pronounced somewhat different to how you may think, based on its spelling - but I cannot repeat its pronunciation now, I've forgotten - but it's definitely not SOUR-SOP). In Spanish the fruit is called guanabana; in Portuguese, graviola.
According to Purdue University's horticulture website:
The fruit is more or less oval or heart-shaped, some times irregular, lopsided or curved, due to improper carper development or insect injury. The size ranges from 4 to 12 in (10-30 cm) long and up to 6 in (15 cm) in width, and the weight may be up to 10 or 15 lbs (4.5-6.8 kg).

I did not think that such a big, heavy fruit could come from a tree, but apparently that is the case. Soursop trees are evergreen, low-branching and slender. The internet is full of websites calling attention to the anti-cancer properties of this fruit.
Who knew?