It’s the plural form, the singaler form is “leaf”。
T: Look at the leaves in winter? Do the trees have any leaves in winter?
（The trees don’t have any leaves in winter.） （板书 The trees ------ leaves in ------.）
T: How about in spring? Do the trees have any leaves in spring? Are the leaves new or old?
（Yes, the trees have new leaves in spring.）
T: What colour are summer leaves? （green）
Do you know summer leaves can make you cool?
So, trees have cool, green leaves in summer. Say it, please.
T: Look at the leaves in fall. Do the trees have cool, green leaves in fall?
The trees lose their leaves in fall.
“lose”， what’s the meaning? Here, it means “失去”。
What colour are fall leaves? （yellow, orange, red）
T: Class, different seasons, different trees and leaves. Can you say the differences about the leaves ?