One of the first topics you come across when starting to study trees is the question of how they manage to lift water all the way to the leaves at the top of the canopy.
Different organs play their part in this system, starting with the roots where water is absorbed into the xylem. Xylem is a network of interconnected cells, which die quickly after birth, so that the cell contents is eliminated leaving a large space for water to enter. New xylem is constantly being created in the roots, trunk, branches and leaves, and this is all connected so that water can pass from one to the other.
But what causes it to rise up towards the leaves? The phenomenon is well described in pretty much any tree biology book you care to pick up (see references page). The answer (as is beautifully described in Ennos’s book ‘Trees’) is that it is pulled from above.
The force which pulls up the water actually starts at the leaves. Cells in leaves need gases to photosynthesise and respire (carbon dioxide and oxygen), and the waxy epidermis (outer layer) is impermeable to gas. So, leaves have small holes called stomata which are pores in the epidermis allowing gas to enter the leaf interior. These holes also allow water vapour to escape from the leaf, and as this water vapour evaporates from the leaf it pulls up the water underneath it by hydrostatic force. Water is strongly attracted to its own molecules (a force known as cohesion), and when they move upwards by evaporation it creates tension pulling more water up. This is known as the ‘cohesion-tension’ theory (Smith et al) and the process is known as transpiration. This is why trees need far more water than their size would suggest – the majority is evaporated from the leaves during transpiration.
As most bonsai enthusiasts know, when you cut a branch, water does not spurt out. So it’s obviously not being pumped from the roots. But you can make water spurt out, if you put a cut branch in a pressure vessel and apply pressure which is equal to the tension that the water was under. Experimentally this has shows stretching forces of over 20 atmospheres (294 p.s.i) (Ennos), evidence which has supported the cohesion-tension theory. There are those who disagree with this as the exclusive mechanism for water movement against gravity – one paper argues that there is an “interplay of several forces including cohesion, tension, capillarity, cell osmotic pressure gradients, xylem-phloem re-circulation, and hydrogel-bound gradients of the chemical activity of water”.ref
Whatever the nuances of the forces involved, the transpiration flow is essential for other processes within the tree – it helps maintain cell turgor (stiffness), maintains solute levels in cells which are needed for metabolism, draws nutrients, plant growth regulators and metabolites up through the tree from the roots via the xylem sap, cools leaves via evaporative cooling, and supplies water to the top of the phloem for the transportation of photosynthates (Smith et al).