Roots and Translocation
Below is a diagram of a cross section of a primary root.
The epidermis is the outside layer of the root and holds everything in place.
Beneath this are parenchyma [pah-ren-ki-ma], they are packing cells and make up the bulk of the root. Generally they are responsible for storing starch and also respiration.
The endodermis is a layer around the vascular bundle, the cells here contain a casparian strip which helps to control water movement into the xylem.
Arranged rather strangely are the xylem and phloem, the xylem transports water, and the phloem transports sugar, which we go into more detail about below.
This is the process of transporting sugar and takes place in the phloem; how we think it travels through them is called the mass-flow hypothesis.
The source is where food is produced, this would be the leaves. They produce glucose which is then converted to sucrose which enter the phloem. This makes the water potential more negative making water from the surrounding xylem enter.
All of this extra material increases the pressure and forces the solution down and through the sieve plate. Then it gets to the sink where the sucrose is moved by active transport into the parenchyma; where it is made into insoluble starch so the water returns to the xylem.
Therefore the solutes move from source to sink, but it is important to note that it can go in both directions. For example, at night when there is no light for the leaves to produce sugar by photosynthesis, energy has to come from storage.
Evidence for this theory comes from two sources: the first is radioactive tracers which shows the direction of flow of sucrose, and second we use ringing experiments.
In the ringing experiment, a ring of bark is scraped away that also removes the phloem. After a while sugar is trying to be transported down the stem but it is stopped by the ring. This means a bulge above the ring forms of sugar. This suggests that sugar moves down the stem in the phloem.
The second method uses radioactive carbon dioxide (where the C is 14C). This radioactive carbon dioxide is put in a bag over a leaf and sealed. The carbon dioxode gets converted into glucose and an x-ray can be taken that will show where this radioative material has moved to.