The phloem vessels (also called the free) are made up of living cells, whose wall has only the cellulosic skeletal membrane typical of plant cells and a thin plasma membrane.
These are highly specialized cells that lose their nucleus during the differentiation process. Its interior is occupied by elaborate sap (or organic sap) and many protein fibers, typical of phloem. The passage of organic sap from cell to cell is facilitated by the existence of riddled plates on the terminal walls of the touching cells.
Through the sieves, the elaborate sap flows from one cell to another, along with thin cytoplasmic filaments, the plasmodesmos.
The holes of the screened plates are lined with callose. Polysaccharide that obstructs the sieves when, in some vegetables, the sieved vessels periodically run out of function. When they return to activity, this callus is undone.
Alongside the sieved tubes are some thin, nucleated cells, called companions, whose nucleus also directs the life of the conducting cells.
The conduction of elaborate sap
The organic sap, elaborated in the parenchyma of the leaves, is thrown into the sieved phloem tubes and carried to all parts of the plant that are not self-sufficient. The transport is mainly oriented to the root, and there may be some movement towards the apex of the developing stem and leaves.
In general, organic materials are translocated to consumer and reserve organs, and movement (ie, reserve organs to growing regions) may be reversed when necessary.
The most widely accepted hypothesis for conducting the elaborate sap is that formulated by Munch and is based on the movement of the entire phloem solution, including water and solutes. It is the hypothesis of mechanical entrainment of the solution, also called the solution's mass flow hypothesis. By this hypothesis, the transport of organic compounds would be due to a rapid displacement of water molecules that would drag the molecules in solution in their motion.
Understanding this hypothesis is made easier by following the model suggested by Munch for his explanation.
Observing the figure, it is concluded that water will flow through osmosis, from bottle A to osmometer 1, and from bottle B to osmometer 2. However, as the solution of osmometer 1 is more concentrated, the flow velocity of water from bottle A to osmometer 1 is larger. Thus, water will tend to flow into glass tube 1 with speed, dragging sugar molecules. As osmometer 2 receives more water, it passes to bottle B. From bottle B, water passes to glass tube 2, toward bottle A. We can match the previous model to a plant:
- Glass tube 1 corresponds to phloem and glass tube 2 corresponds to xylem;
- Osmometer 1 corresponds to a leaf parenchyma cell and osmometer 2 corresponds to a root cell;
- Bottle A represents the leaf, while Bottle B represents the root;
- Leaf parenchyma cells perform photosynthesis and produce glucose. The concentration of these cells increases, which causes them to absorb water from the ribs xylem. Excess absorbed water is shifted to the phloem, dragging sugar molecules toward the consumer or reserve centers.