Learn Italian: La pianta tropicale, le cui foglie sono rosse, cresce nella serra. - The tropical plant, whose leaves are red, grows in the greenhouse.

When using cui to mean whose, the definite article right before it has to agree with the object being possessed, not the owner! Even though the owner is singular (la pianta), the things it possesses are plural (le foglie). Therefore, we use the feminine plural article le.

What is the exact word order when using cui for possession?

The structure for translating whose is always: definite article + cui + noun being possessed. Here, that gives us le + cui + foglie. The pronoun cui sits snugly between the article and the noun.

I remember learning the word fogli. Is foglie related to that?

Yes, but they have distinct meanings based on gender! The feminine plural le foglie refers specifically to the leaves of a plant or tree. The masculine plural i fogli refers to sheets of paper.

Why does the sentence say nella serra instead of in la serra?