● Food, `color{brown}("primarily sucrose,")` is transported by the vascular tissue `color{brown}("phloem")` from a `color{brown}("source to a sink. ")`

● Usually the `color{brown}("source")` is understood to be that part of the plant which `color{brown}("synthesises the food,")` i.e., the leaf, and `color{violet}("sink,")` the part that needs or `color{violet}("stores the food.")`

● But, the `color{violet}("source and sink")` may be`color{brown}(" reversed")` depending on the `color{brown}("season,")` or the plant’s needs.

● Sugar stored in roots may be `color{brown}("mobilised")` to become a `color{violet}("source of food")` in the `color{brown}("early spring")` when the buds of trees, act as sink; they need energy for growth and development of the `color{violet}("photosynthetic apparatus.")`

● Since the `color{brown}("source-sink relationship")` is variable, the direction of movement in the `color{violet}("phloem")` can be upwards or downwards, i.e., `color{brown}("bi-directional.")`

● This contrasts with that of the `color{brown}("xylem")` where the movement is always `color{brown}("unidirectional,")` i.e., upwards.

● Hence, unlike `color{brown}("one-way flow of water")` in `color{violet}("transpiration, food in phloem sap")` can be transported in any required direction so long as there is a source of sugar and a sink able to use, store or remove the sugar.

● `color{violet}("Phloem sap")` is mainly `color{brown}("water and sucrose,")` but other `color{brown}("sugars, hormones")` and `color{brown}("amino acids")` are also transported or `color{brown}("translocated")` through `color{violet}("phloem.")`

● The accepted mechanism used for the `color{violet}("translocation of sugars")` from source to sink is called the `color{brown}("pressure flow hypothesis.")`

● As `color{brown}("glucose")` is prepared at the source (by `color{violet}("photosynthesis")` ) it is converted to `color{brown}("sucrose")` (a dissacharide).

● The sugar is then moved in the form of `color{violet}("sucrose")` into the `color{brown}("companion cells")` and then into the living phloem `color{brown}("sieve tube cells")` by `color{brown}("active transport.")`

● This process of loading at the source produces a `color{brown}("hypertonic condition")` in the `color{violet}("phloem.")`

● Water in the adjacent `color{violet}("xylem")` moves into the `color{violet}("phloem")` by `color{brown}("osmosis.")`

● As `color{violet}("osmotic pressure")` builds up the `color{violet}("phloem sap")` will move to areas of lower pressure.

● At the `color{brown}("sink")` osmotic pressure must be reduced.

● Again `color{brown}("active transport")` is necessary to move the `color{violet}("sucrose out of the phloem sap")` and into the cells which will use the sugar – converting it into `color{violet}("energy, starch, or cellulose.")`

● As sugars are removed, the `color{brown}("osmotic pressure decreases")` and water moves out of the `color{violet}("phloem.")`

● To summarise, the movement of `color{violet}("sugars in the phloem")` begins at the `color{brown}("source")`, where sugars are `color{brown}("loaded")` (actively transported) into a `color{violet}("sieve tube.")`

● Loading of the `color{violet}("phloem sets")` up a `color{brown}("water potential gradient")` that facilitates the mass movement in the `color{violet}("phloem.")`

● `color{violet}("Phloem tissue")` is composed of `color{brown}("sieve tube cells,")` which form long columns with holes in their end walls called `color{violet}("sieve plates.")`

● `color{brown}("Cytoplasmic strands")` pass through the holes in the `color{violet}("sieve plates")`, so forming continuous filaments.

● As `color{brown}("hydrostatic pressure")` in the phloem sieve tube increases, pressure flow begins, and the sap moves through the `color{violet}("phloem.")`

● Meanwhile, at the `color{violet}("sink")`, incoming sugars are `color{brown}("actively transported")` out of the `color{violet}("phloem")` and removed as `color{violet}("complex carbohydrates. ")`

● The loss of solute produces a `color{brown}("high water potential")` in the `color{violet}("phloem")`, and water passes out, returning eventually to `color{violet}("xylem.")`

● A simple experiment, called `color{brown}("girdling,")` was used to identify the tissues through which `color{violet}("food is transported.")`

● On the trunk of a tree a ring of bark up to a depth of the `color{brown}("phloem layer,")` can be carefully removed.

● In the absence of downward movement of `color{violet}("food the portion")` of the bark above the ring on the stem becomes `color{brown}("swollen")` after a few weeks.

● This simple experiment shows that `color{violet}("phloem")` is the `color{violet}("tissue responsible")` for `color{brown}("translocation of food")`; and that transport takes place in one direction, i.e., towards the roots.