The movement of water up through the plant's xylem, into the leaf and then into the atmosphere is known as the transpiration stream. The loss of water from a plant leaf will be greatly affected by external factors such as temperature, wind velocity and humidity. All of these factors will affect the rate at which water evaporates into the air. The higher the temperature the faster water evaporates. The greater the rate of air movement over the surface of a liquid the greater its rate of evaporation. The lower the humidity, that is the amount of water vapour already present in the air, then the faster the rate of evaporation.

Organic molecules are produced as a result of photosynthesis mainly in the palisade mesophyll of the leaf. These are usually stored in the chloroplasts during the daytime and are translocated at night through the phloem to other regions of the plant where resources are needed for storage, growth and respiration. Glucose molecules are stored in the chloroplasts as starch molecules, and these are converted back to glucose during nighttime and moved to the endodermal cells surrounding the vascular bundles of the leaf. The glucose is converted into sucrose and is loaded into the phloem sieve cells. These cells are highly specialised to form transport functions and require a companion cell to produce materials to maintain their function. The companion cells are cytoplasmically connected to the sieve cells by the plasmodesmata which are cytoplasm channels running through the cell walls.

 

Some companion cells are modified to from transfer cells. These cells pump Hydrogen ions out through the cell membrane and these flow rapidly back into the cell through specific carrier proteins that will only work if sucrose is transported at the same time as the hydrogen ions.

The loading of the phloem within the leaves causes the water potential of the sieve cells to drop to very low levels. This results in an osmotic influx of water. As the water flows into the confined space of the sieve vessels its pressure causes movement from one vessel, through the pores of the sieve plate and into the next vessel.

The idea of mass flow is shown in the diagram above. The osmotic flow of water into the sieve vessels in the leaves cause a flow which carries the sucrose to the roots, where the sucrose is unloaded and converted into insoluble starch which increases the water potential in the root, the excess water is then drawn into the xylem and moved back up to the leaf. The water lost by evaporation from the leaves is made up by absorption from the root hairs. As the xylem carries water up to the leaves, mineral ions absorbed by diffusion and active transport in the roots will be transported along with it.

 The reason that the phloem must be alive to be able to carry out its transport function is that the mass-flow principle used by the phloem depends upon osmotic uptake of water in the leaves and for this the cells of the phloem need cell membranes. Aphids feed upon plant sap by inserting their hypodermic like mouth parts into individual sieve vessels and the pressure of the fluids in these actually pumps the food through the animals gut. Some experiments on transport of organic materials have been done by allowing aphids to begin feeding and then decapitating them to leave the mouth parts inserted in the plant. The fluid which oozes out is then removed and analysed, for example for the presence of introduced radioactive tracers such as ¹⁴C. This has allowed us to examine the bidirectional flow of materials along the phloem and the speed at which materials are translocated.

 The problem with a pressurised transport system is that if it springs a leak then it must be rapidly sealed. In the human system we use a complex series of chemical factors to enable us to convert soluble fibrinogen into insoluble fibrin which forms a network over the cut. The plant uses a pressure sensitive protein so that if the pressure in the sieve vessels suddenly drops the protein coagulates in the sieve plate pores and effectively seals them off.

Münch’s mass flow experiment:

The diagram above illustrates the arrangement of the apparatus used by Münch. Two semi-permeable membrane bulbs are connected by a fine bore glass tube. Each bulb is placed into a beaker of distilled water. The glass tubing is filled with distilled water as is the right hand bulb. The left hand bulb is filled with a concentrated sucrose solution. Water is sucked into the left hand bulb by osmosis, as the pressure increases a flow of water is set up along the glass tube and into the right hand bulb. The water escapes from this into the beaker and then runs along the tubing connecting the two beakers. As the flow of liquid from the left bulb to the right bulb continues the sucrose solution is pushed along and finally appears in the right hand bulb. The beaker on the left represents the leaf where sucrose is loaded into the sieve cells. The fine bore glass tubing connecting the bulbs represents the phloem, the right hand beaker represents the root and in a real root the sucrose would be pumped out and converted into starch in the root cells. The tube connecting the two beakers represents the xylem carrying water from the root to the leaf.

It was once thought that the simple mass flow hypothesis originally suggested by Münch could not achieve the flow rates which had been observed experimentally, but recent evidence gained by the measurement of pressure gradients within the phloem have shown that these are great enough to explain the flow rate.