There were no Coles or Woolworths supermarkets in the 1700's. No refrigeration to provide out of season foodstuffs either.

Food was much more seasonal, and much more expensive in real terms compared to today. Consequently many cottages would have herb and vegetable gardens where they could grow part of their own food either for consumption or trade, an example of which exists around Cook's cottage.

In this case we're seeing the leaves of a cabbage plant. This has a dual connection with Cook, who used pickled cabbage (also known as Sauerkraut) to help ward off scurvy on his voyages. Cabbage is in fact a rich source of vitamin C.