Primary and secondary growth
The way a plant grows depends on where the meristems are located, that is, the embryonic tissues. The apical meristems are located at the ends of the thinnest roots and in the bud buds, these tissues continue to produce new cells that make roots and twigs grow in length. This stretch is called primary growth and allows the roots to grow and branch in the ground and the young shoots to develop and branch upwards. In many plants, such as herbaceous plants, only a primary growth occurs; the woody plants instead grow not only in length but also in thickness (secondary growth).
Lsecondary growth it therefore causes the stem and roots to become red and is due to the activity of the lateral meristems: tissues that form a cylinder that runs along the roots and stems along the entire length. These lateral cylindrical meristems continue to produce new cells both inwards and outwards. In woody plants such as primary and secondary growth trees occur simultaneously, but in different places. Primary growth occurs in the buds and the apices of the roots; the secondary one instead mainly in the slightly older parts where roots and young stems are swelling.
When the vascular change between primary xylem and primary phloem appears, secondary growth begins. Subsequently, the change forms a sort of ring between the xylem (which remains inside) and the phloem towards the outside. The exchange cells will continue to reproduce rapidly producing wood inwards and book outwards.
The xylem (wood) is made up of cells or groups of elongated tubular cells. These cells develop a lignified secondary cell wall that becomes completely rigid, when they mature they die: cytoplasm, organelles and cell membranes disintegrate and the cell no longer opposes osmotic resistance to the flow of water that flows through it. The lignified secondary cell wall ensures that the cell retains its tube shape even after death.
The cells of the phloem (book) present on their walls numerous cribrose areas (with numerous perforations of the cell wall) that put these cells in communication with each other. In the phloem of the angiosperms, these cells are arranged in longitudinal rows, called the cribrose tubes, made up of cells called elements or members of the cribroso tube (it is therefore an arrangement similar to that of the xylem vessels). Where one element of the cribral tube comes into contact with the next, there is a cribral plate that allows the passage of liquids from one element to another.