The organization of the tissues of the stem differs between dicots and monocots.
The Woody Dicot Stem
The drawing shows a sector of a cross section through a 5-year old twig from a basswood tree (Tilia). The stem has three areas:
Fig. 188.8.131.52 Dicot stem
- Cork - The outer part of the bark is protected by layers of dead cork cells impregnated with suberin. Suberin is waxy and cuts down water loss from the stem. But suberin is as impervious to air as it is to water. The gas exchange needs of the living cells beneath the cork are met by openings in the cork called lenticels.
- Cortex - Layers of parenchyma cells. These store food (as they do in the root). In the very young stem (before cork has formed), they may have chloroplasts and carry on photosynthesis.
- Cork cambium - In older stems, a meristem forms between the cork and cortex. Mitosis of its cells produces more cork.
- Expanded pith rays - Regions of parenchyma that store food.
- Phloem - Bundles of sieve tubes surrounded and supported by sclerenchyma. Translocation of food through the stem takes place in the sieve tubes.
During the growing season, mitosis in this band of meristematic tissue produces new phloem to the outside and new xylem to the inside.
Xylem makes up the wood region. The xylem vessels made in the spring, when water is plentiful, have larger diameters than those made later in the season. No xylem is made during the dormant season. The visual contrast between the late summer xylem of one season and the spring xylem of the next creates the annual ring. Xylem serves two functions:
- transport of water and minerals up the stem
In older stems only the most recent rings of xylem are active in transport. These make up the sapwood. The older xylem - the heartwood- support the weight of the tree.
In young stems, the pith, which is made of parenchyma, stores food. The pith disappears in older stems.
The Monocot Stem
Fig. 184.108.40.206 Monocot stem
The photograph (courtesy of Turtox) shows the organization of tissues in the corn (maize) stem, a typical monocot. The corn stem consists of:
- an external rind
- an interior filled with pith.
Vascular bundles are scattered through the pith.
Each vascular bundle contains:
- a layer of sclerenchyma that provides support
- a bundle of phloem containing
- sieve tubes used for food transport
- their companion cells.
- four xylem vessels
- a group of xylem tracheids
- both carry water and dissolved minerals up the stem