J.M., you might wonder why the τὰ is repeated after ποιήματα. It's because the perfect participle γεγραμμένα is being used as an adjective modifying ποιήματα, and any time you put the adjective after its noun, you need to repeat the article. (Example: ὁ ἀγαθὸς ἄνθρωπος = ὁ ἄνθρωπος ὁ ἀγαθος.)
The writer could have put the entire participial phrase before the noun, to look like this: τὰ περὶ τῆς φύσεως γεγραμμένα ποιήματα. In that case, you don't have to repeat the article. So (literally) "the poems the written about nature" = "the written-about-nature poems"!