Many people know "quite critical". What leaves do the "leaves" here refer to?

According to my understanding, a leaf represents the arrival of autumn, and that leaf should refer to buttonwood leaves.

I often watch TV plays and novels. In ancient times, they had no clocks. They only know what solar terms are at this time according to the color of leaves. Many leaves represent solar terms in different periods, and the leaves of buttonwood just represent the beginning of autumn. I heard my grandmother tell me that they couldn't afford a clock at that time, so they just planted a few representative solar terms to record and remind themselves of what solar terms they were in.

When I was in primary school, I planted a buttonwood tree downstairs, which was very tall. Every time I look at it at the window, with the passage of time, I grow up slowly, and the buttonwood tree also grows up. Whenever autumn approaches, its leaves turn yellow and beautiful. I like me very much. I often go downstairs to pick up the leaves of buttonwood, which can be used as specimens. It has been with me for many autumns. Now that I have grown up, I don't often see my buttonwood, and sometimes I miss it. Autumn is coming, I will run to play under the tree and miss us at that time. I also heard that lovers' wishes under the buttonwood tree will come true and they will be very happy. When I am old, I will plant a buttonwood tree near my home. Every day after dinner, I will take a walk under the buttonwood tree. I like buttonwood, like its leaves, and you like the feeling of autumn.

Quite critical, a leaf can tell that it is autumn. I think it is easy to understand. I'm such a stupid person. Of course I know. I know.