Water from soil enters the plant from the root cells, taken up the stem in the xylem, enters the leaf and diffuse through the mesohpyll cells.
On the other hand, carbon dioxide cannot be taken up from the roots. Therefore, the carbon dioxide needed to enter the leaf from outside the leaf, through the tiny pores in the leaf called stomata, and diffuse inside the leaves. The carbon dioxide cannot directly enter the leaf without passing through the stomata, this is because on the leaf, there's a layer for protection called cuticle. They prevent water from evaporating away, thus, carbon dioxide also cannot pass through the cuticle.
