It's a determiner. That's kind of like an adjective, only with more limited application. Incidently, the articles "a", "an" and "the" are also determiners, which is why you can "these" (as well as "some", "whose", "much" and a whole bunch of other similar words) wherever you can use "the".
Rommie
PS. Can someone tell me what "diagram sentences" (as