Organic Materials in Plants: The green plants manufacture food in the leaves. From the leaves, the sugars need to be moved to the non-photosynthetic parts of the plants such as roots, stems and flowers for storage or consumption.

The manufactured food substances (sugars) are transported from the leaves to the other parts of the plant body by phloem tissue (mainly sieve tubes) as its structure is well suited for this function.

The transport of carbohydrates, mostly in the form of sucrose (sugar), from the leaves to all other parts of the plant is called translocation. In addition to sugars, the phloem also transports amino acids, growth hormones, and vitamins.

The mechanism of the translocation of these substances thorugh the sieve tube cells of the Phloem is not yet clearly understood. Several hypotheses have been proposed to explain this mechanism. One of these hypotheses is the mass flow hypothesis (Fig.  12.9). many plant physiologists favour this hypothesis.

pressure-flow-hypothesis

Fig. 12.9 pressure flow hypothesis.

According to the mass flow hypothesis,

The sugars in solution can move freely through the Sieve tubes from the leaves to all non-photosynthetic parts of the plants.This movement is because of the difference in the concentration gradient at the place of manufacture of sugar i.e., the leaves and the other parts of the plant which need nourishment to carry out their activities.

As the food is manufactured in the leaves, the sieve-tubes in their veins are loaded with sugar. More water therefore enters then by osmosis. Consequently they have a high sugar pressure. Other parts of the plants have low concentration of sugar pressure. So the sugar will move from a region of high concentration (source) to the region of low concentration(sink).

The phloem stream, carrying the manufactured   food, moves in the direction opposite to that of the transpiration stream. One objection against the mass flow hypothesis is that the phloem often transports substances in the opposite direction too i.e., up the stem to provide nourishment and energy to the leaves and shoots for their growth and also to the developing seeds and fruits.

But this objection to the mass flow hypothesis does not hold good if we recall the structures of the phloem tissue. The phloem tissue consists of many sieve tubes, not just one, and the fluids can move in opposite direction in two different but closely situated sieve-tubes. Here too, the movement of fluids in opposite direction would also depend on opposing pressure gradients in the two ends of sieve-tubes.