how does water go up tall trees, through the xylem, like the sequoia tree, if the maximum hight of water being pumped upwards is of ≈ 10.3 metres high, due to pressure

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valentin68 | College Teacher | (Level 3) Associate Educator

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There are two different phenomena that are responsible for water being raised into very tall trees like sequoia.

First of them is that water raise to a certain height h in a narrow tube by the phenomena of capillarity. In a few words, because of the surface tension, and because the force of adhesion of water molecules to the tube wall is bigger than the force of cohesion between water molecules themselves, water forms at its surface in the tube a concave meniscus (it is said that water wets the walls of the tube-see the figure below). This way, the resultant of the adhesive forces pulls up from the surface of water the entire column. The height to which water can raise in a tube of radius R is given by (Jurin law)

`h = (2*gamma*cos(theta))/(ro*g*R)`

where `gamma =0.073 N/m` is the coefficient of superficial tension for the water-air interface, `ro = 1000 (kg)/m^3` is the density of water, `theta` is the so called wetting angle (`=14 degree` ) for water in glass tubes and `R=10^-6 m` is a plausible radius value for the tree xylem.

With the above values we obtain `h=14.4 m`

The second phenomena that makes water raise in high trees is the water cohesion force that exist between water molecules. Water forms a continuous tube along the xylem from tree roots to its leaves. When water evaporates at the surface of one leaf it creates a negative pressure, that pulls up only a bit the entire column of water in the tube because of the cohesive forces between water molecules.

It is true that a difference in pressure of one atmosphere is equivalent only to 10.3 meters high water column, but in the case of a tree, each leaf is creating its own negative pressure and because the leaves are situated at different heights between each other water can raise step by step to very high distances from the tree roots.

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