- STRUCTURE OF THE PLANT STEM
THE GENERAL STRUCTURE OF A PLANT STEM
The stem is the main structure above the ground and it bears and supports all other plant structures. The structure of the stem can be studied from two angles:
THE EXTERNAL STRUCTURE
The external structure refers to the structure one can observe on the stem as it stands, before being subjected to any interference. The point where the leaves are attached on the stem is called the node. The length of stem between the nodes is called the internode. The axil is between the leaf and the stem. Branches arise from a bud that develops in this position called the axillary bud. At the end of the stem is the growing point – the terminal bud, which gives rise to more stem. When a leaf is shed, a scar is left and this is called the leaf scar. See Figure 1.
THE INTERNAL STRUCTURE OF THE STEM
A stem will always possess four essential types of tissue which are as follows:
- A protective surround;
- Conductive tissue for conducting water and manufactured foodstuffs;
- A packing and storage tissue; and
- Tissue providing structural support.
Some stems possess a fifth type of tissue, which serves to divide and add to the other tissues. This meristematic tissue is called the cambium.
Figure 1: A typical stem.
Figure 2: A Cross-section through a terminal bud.
A PROTECTIVE SURROUND
In woody stems this takes the form of cork which is commonly called the bark. In herbaceous stems (non-trees) and young woody stems the protective surround is a single layer of cells called the epidermis. Cork is many cells thick and is continuously being added to by the cork cambium.
Here two types of tissues are involved: the xylem and the phloem.
- The xylem is made up of xylem vessels (previously described);
- The phloem is made up of sieve tubes (previously described); and
- The xylem and phloem are found together in the vascular bundles.
PACKING AND STORAGE TISSUE
This tissue is made up of parenchyma cells; these are living spherical cells.
This is usually tissue containing dead lignified cells, except for collenchyma cells. The other supporting tissue is the sclerenchyma. See Figure 3.
THE STRUCTURE OF A MONOCOTYLEDONOUS STEM
In a monocotyledonous stem the vascular bundles are scattered throughout the stem. There is generally no cambium between the xylem and phloem and in most cases these stems are herbaceous. See Figure 4.
THE STRUCTURE OF A DICOTYLEDONOUS STEM
In a dicotyledonous stem the vascular bundles are arranged in a ring. Between the xylem and phloem is a thin meristematic tissue called the cambium. As the stem grows older the cambium forms a complete circle, and eventually so does the xylem and phloem. The xylem is added to each year and forms the annular rings from which trees can be aged. The cork cambium develops and produces a layer of cork to protect the tree. The phloem eventually becomes a thin ring of tissue around the stem and forms part of the bark. See Figure 5.
The stem serves two main functions:
- Firstly to provide support to the plant and support other surface structures, Secondly to transport water and minerals from the roots to the rest of the plant and manufacture sugars from the leaves to the rest of the plant.
- The stem can also act as a place for the storing of foodstuffs.
- The xylem transports water and minerals and the phloem transports manufactured foodstuffs.
Figure 3: A cross-section of a dicotyledon stem.
Figure 4: A cross-section through a young monocotyledon stem.
Figure 5: A comparison of an old woody stem and a young Dicotyledon stem.
3. DEVELOPMENT OF THE PLANT STEM
The tip of the stem is made up of the apical meristem which is continuously undergoing cell division to produce more cells. The cells then enlarge or increase in size and subsequently differentiate into specialised cells such as xylem and phloem. These specialised cells then mature. In monocotyledons, this is the sum total to the story of stem development.
However, in many dicotyledons there are further developments in the stem. Secondary thickening occurs. This is where the cambium which is not usually present in monocotyledons, divides and continuously produces more xylem and phloem tissue. The latter is pushed to the periphery of the stem and can be removed by ring barking. The xylem is added to seasonally, and forms the annual rings from which trees can be dated. Cork is also formed from the cork cambium. Diagram 4.5 shows a diagrammatic representation of a longitudinal section through a stem showing its development. The diagram also shows similar cross sections through the stem at various stages of development. Such plants that have secondary growth form cambium are woody plants i.e. trees and shrubs. Not only dicotyledons are woody in the conventional sense. Students must be aware that the apical meristem is contained in the terminal bud.
4. MODIFIED STEMS
You must appreciate that not all stems grow vertically or are even supporting structures. Some stems have been modified by natural evolution and are called modified stems.
There are four main types of Modified Stems:
- Tubers; and
A stolon is a runner, such as strawberries and some grasses. This is a stem that grows horizontally over the surface of the ground. Vertical stems propagate from the nodes.
This is also a runner but runs below the surface of the ground.
A tuber is a swollen stem used as a storage organ, such as the potato. On the potato, the “eyes” are in fact the nodes from which a new vertical stem and new plant can originate.
Figure 6: The Development of a Woody Dicotyledonus
This diagram shows the growth of a twig with cross sections at intervals down the stem.
An example of the bulb is the onion. This is a short stem surrounded by many leaves compactly arranged. The leaves are thick and are modified as storage organs. The leaves in the bulb do not make their own food and are not green.
Figure 7: An Onion bulb.