What is capillary action and why is it important

Capillary action can be defined as the ascension of liquids through slim tube, cylinder or permeable substance due to adhesive and cohesive forces interacting between the liquid and the surface, another possible definition is that it's the ability of a liquid to flow in narrow spaces without the assistance of, or even in opposition to, external forces like gravity.

Without capillary action, the water level in all tubes would be the same. Smaller diameter tubes have more relative surface area inside the tube, allowing capillary action to pull water up higher than in the larger diameter tubes.

biologically it's important in plants and trees, Water is brought high up in trees by branching; evaporation at the leaves creating depressurization; probably by osmotic pressure added at the roots; and possibly at other locations inside the plant, especially when gathering humidity with air roots