Capillary force theory to explain ascent of sap was proposed by
Capillarity Theory: It is given by Boehm (1809). According to this theory, water is translocated because water molecules adhere to the surfaces of small, or capillary, tubes. This adhesion causes water to somewhat creep upward along the sides of xylem elements. In glass tubes this upward movement is visible as the curved or crescent-shaped (concave) meniscus. However, it was shown that capillarity (or capillary rise) alone in tubes of similar diameter as that of a xylem element raises water less than 1 meter.
Hopkins (1999) explained that the relationship between the rise of water in a capillary tube and the size of the tube is inversely proportional. This means that the thinner is the tube, the higher will be the rise of water.