What's the difference between 잎 and 이파리?

Today I came across the word 이파리 in a story on the Korean Lab Web site entitled, "동네 한 바퀴." I became curious about the word and looked it up.

이파리 is defined in my Korean-Korean dictionary as "잎의 낱개," which means "a leaf." Actually, the literal meaning of the definition is "one of (a number of) leaves," which implies that 잎 is plural, even though my Korean-English dictionary defines 잎 as "a leaf" and then gives examples of it being used in the plural.

Does 잎이 예쁘다 mean "the leaf is pretty" or "the leaves are pretty"? It is a little confusing, especially since the writer of "동네 한 바퀴" made 이파리 plural by writing it as 이파리들. Actually, I think the writer used 이파리들 instead of 잎 because it sounded cuter and more appealing to children.

By the way, the Korean word for "grass" is 풀, and the Korean word for "a blade of grass" is 풀잎. Also, the Korean word for "flower" is 꽃, and the Korean word for "flower petal" is 꽃잎. Therefore, why can we not refer to a single leaf as an 잎잎?