Plants have evolved special tubular structures that allow them to carry water from roots in the soil to growing shoots and leaves through a process called capillary action. How capillary action works is best understood by knowing the elementary physics of molecular cohesion and surface tension in water and a corollary phenomenon called miniscus.
Molecular Cohesion and Surface Tension
The molecules of water are polar, like those of a magnet. The term “molecular cohesion” describes the mutual attraction of water molecules. Molecular cohesion produces a form of tension on the surface of water. Surface tension is what makes raindrops round, and it is the reason why water forms beads when it is poured onto glass.
Water in a glass appears flat except at the edge. The edge of the water clings to the side of the glass, rising in a slight curve. This slightly elevated edge of the water is a “meniscus.” Water in a glass produces a concave miniscus; if the water is filled carefully to the very top so that the miniscus can go no higher, the surface tension of the water forms a convex meniscus. Water in a glass is barely concave. But if it is put in a narrow tube, the surface of the water becomes more concave. If the water is put in a tube so thin that the meniscus on each side of the concave curve can touch, the water produces another menisus and the level of the water rises. This is capillary action.
The primary cells of plants, including those forming the vascular system that carries water through capillary action, are made of cellulose. The “ose” at the end of cellulose means sugar. Plant photosynthesis produces cellulose from water, carbon dioxide and sunlight. Like water, cellulose cells are polar. Water sticks to cellulose just as it does to glass on the interior of glass tubes.
Xylem are vascular tissue with a tubular structure found in woody plants and trees. Capillary action in xylem is aided by transpiration, the evaporation of water from pores in leaves and plant surfaces. Transpiration causes miniscuses necessary for capillary action to form in the interior walls of the xylem. Capillary action is also aided by the upward pressure of water in the roots. The roots obtain water from the soil by osmosis.
Phoem are the cells in non-woody plants that carry water through capillary action. The long, tube-shaped cells of phoem are joined end-to-end. The cells have no nuclei. The cells that connect them at the end are shaped like a sieve with numerous small holes that allow fluids to pass from one cell to the next.