Halliday & Matthiessen (2004: 575, 576):
The general sense of hyponymy ia ‘be a kind of’ … There is a similar relation in the extending domain. This is meronymy — ‘be a part of’ … There is no very clear line between meronymy and hyponymy, especially with abstract terms; and a given set of items may be co-hyponyms of one term, but co-meronyms of another … But since either relationship is a source of lexical cohesion it is not necessary to insist on deciding between them.