They usually have leaves, and their growing tip or shoot apex has small stubs on its side, which develop into leaves. The stem has several functions. It acts as a support for the leaves, and transports nutrients to the leaves and food to the roots. In some plants, such as cacti, the stems also photosynthesize and store food.
Stems comprise several parts.
In the middle is the pith, a region of undifferentiated cells which, together with the cortex, form a base for the other layers to grow in. Surrounding the pith is either a ring or a series of bundles containing the cells that transport fluids. This is known as the vascular tissue. Outside of this is a layer of cells known as the cortex and then the epidermis, the skin of the stem, which is modified into bark in woody plants. The vascular system is made up of xylem, which transports water and nutrients from the roots to the leaves, and phloem, which transports food to the roots and the stem.