The functions of the stem
The stem integrates roots and leaves, both structurally and functionally. In other words, besides constituting the physical structure where roots and leaves are inserted, the stem performs the functions of conduction of water and mineral salts from roots to leaves, it's from conduction of organic matter from leaves to roots.
Young stems have chlorophyll cells and are lined with a uni-stratified epidermis, that is, formed by a single layer (stratum) of cells. Plants that show little growth in thickness, such as grasses, for example, also have stems coated by epidermis and it may also present on itself, externally, a protective cuticle.
Already in plants that grow very thick, becoming shrubs or trees, the epidermis is replaced by a complex lining, formed by various tissues. The outermost tissue is made up of dead cells, which give the rough and opaque appearance to tree trunks. This multitecid coating, called periderm, accompanies the thick growth of the trunks.
Stems are usually aerial structures that grow vertically in relation to the ground. There are, however, stems that grow horizontally, often underground.
Underground stems can be distinguished from roots because they have vegetative buds or buds from which branches and leaves can develop.
The stem buds are formed by groups of meristematic cells, capable of actively multiplying by mitosis. A set of meristematic cells forms a meristem, which is why stem buds are also called stem meristems.
At the apex of the stem (and each branch) there is always an apical bud (or meristem), which allows for growth in extension thanks to the multiplication of meristematic cells. As the stem grows, they differ laterally, regions where leaves and axillary (or lateral) buds appear. The regions where the leaves and buds are inserted are called nodes and the spaces between the nodes are called internodes.
Axillary buds are meristems located on the stem, close to the angle formed between the leaf and the branch, which botanists called Leaf armpit. Axillary buds remain inactive for a certain period, called dormancy after which they can enter activity, leading to lateral branches.