Why is Holly evergreen all year round?

The reason why Holly is evergreen all the year round:

Because its root system is very developed, there are so many roots in plants, which mainly have the following three functions:

First, plants must have strong roots if they want to be blown down by strong winds. So the roots of plants play a fixed role.

Second, in the sun, the green leaves of plants should undergo photosynthesis, which requires water to participate, and water should be absorbed from the soil with roots.

Third, plants still need a lot of nutrients, and nutrients also need roots to find in the soil. The nutrients it needs are basically absorbed by roots, so it can be evergreen in the case of little sunshine in winter.