The main function of the epidermis is protection. The spongy, inner part of the stem is called the pith. The pith is used to store food. In the pith there are a number of hollow tubes, called vascular bundles. These tubes allow the movement of liquid food from the leaves to the roots and carry water and dissolved minerals from the roots to the leaves.
Herbaceous plants produce completely new stems each year.
The approaching cold weather causes the stems to die back to the ground. Some herbaceous plants survive periods of cold by forming underground bulbs, or tubers used for storage. Many herbaceous plants complete their life cycles within one growing season and the whole plant dies, even the roots. These annuals produce seeds that will form new plants the next year. The stems of herbaceous plants remain upright because of the structure of the cells in the stem.