Capillarity Theory (Physical Force Theory)

b. Capillarity theory (physical force theory) : The capillarity theory, also called the physical force theory, was proposed by Bohem in 1863. It attributes ascent of sap entirely to physical forces acting on dead, hollow xylem cells, with no living activity required at all.

The theory draws on everyday capillary phenomena: a wick dipped into an oil lamp draws oil upward by capillarity, and water in a narrow straw dipped into a container of water rises to a certain height above the surrounding water level, purely because of capillarity, with the height reached depending on the diameter of the straw (narrower tubes draw water higher). Capillarity itself arises from the combination of surface tension together with the cohesive force between like water molecules and the adhesive force between water molecules and the walls of the tube. The theory treats each xylem vessel or tracheid, with its narrow hollow lumen, as directly comparable to such a capillary straw: a continuous water column is held up inside the xylem by the combined cohesive and adhesive forces between water and the lignified xylem wall, and it is this capillary action that raises water — though, on its own, only to a height of a few centimetres.

Objections / limitations of capillarity theory: A few important objections are:

i. For true capillary rise, the capillary tube (the xylem) would need to be continuously and completely hollow from one end to the other, but tracheids actually have closed end-walls, interrupting any truly continuous capillary column.

ii. The lower end of a genuine capillary tube must be in direct contact with the water it is drawing up, but in a real plant there is a barrier of root cortex tissue lying between the xylem and the soil water, so the xylem is never in that kind of direct contact. …