A stem has leaves at regular intervals and a terminal bud at the growing point. The region of the stem from which the leaves are borne is called the node and the length of the stem between the nodes is the internode. In general the stem is erect and green, but may be horizontal as in runners; underground as in rhizomes; very short and never showing above the ground as in bulbs and corms; long, thin and weak as in climbing plants; or stout and thick as in trees.
The living cells in stems get their supply of oxygen from the air through pores called stomata and lenticels in their epidermis. Older stems are supported by woody and fibrous tissues which are added layer by layer thereby increasing their thickness. Running through the stem are tubes which conduct water from the soil up to the leaves and food from the leaves to the various parts of the plant.
The stem supports the structure of the shoot; spaces out leaves so that they receive sufficient air and sunlight; allows the conduction of water from the soil to leaves and food from leaves to the other parts of the plant; holds the flowers above the ground thus assisting pollination by wind and insects and supports photosynthesis (green stems).
Forms of Stem
There are different types of stem. The stem may be erect, rigid or strong.
- If the stem is unbranched, erect, cylindrical and marked with scars then it is referred to as caudex e.g. palm.
- If the stem is branched with solid nodes and hollow internodes it is referred to as culum e.g. bamboo.
- In cases like onion, the aerial shoots bear flowers and hence are referred to as Scape.
- If the stem trails on the ground they are called as prostrate e.g. Oxalis.
Modification of StemStems of different groups of plants show many modifications. These modifications are, in fact, adaptive features to suit various special functions which help the plant in its survival and reproduction.
Structures such as bulbs, corms, rhizomes and stem tubers, which exist underground, are called underground stems. These are modifications which help in food storage and vegetative reproduction of plants. The storage organs of many plants are frequently stems with lateral buds that give rise to new plants, growing from the parent stem.
These are condensed shoots with fleshy leaves. The stem is very short and never grows above the ground. The internodes are short; the leaves are very close together and they overlap. The outer leaves are scaly and dry and protect the inner ones which are thick and fleshy with stored food. In the Harmattan lily Hippeastrum, and Crinum lily, the bulb is formed by the bases of the leaves which completely encircle the stem. In the onion bulb, all the storage leaves are cylindrical and are not part of the leaves appearing above the ground. In the leaf axils of bulbs are lateral buds which can develop into new bulbs and shoots.
Fig: Section through the Bulb of Lily (Hippeastrum)
Some plants store food, not in special leaves or leaf bases but in the stem, which is very short and swollen, forming a corm. When the foliage has dried off, the leaf bases, where they encircle the short stem, form protective scaly coverings. A familiar corm is that of the ornamental cocoyam (Caladium). Since a corm is a stem, it has lateral buds which can grow into new plants. The stem remains below the ground all its life, only the leaves and flower stalk arises above the ground.
Fig : Corm of Coco Yam
- Rhizomes and Stolons
In rhizomes, the stem remains below the ground but continues to grow horizontally. The old part of the stem does not die away as in bulbs and corms, but lasts for several years. In Canna, the terminal buds turn up and produce leaves and flowers above ground. The old leaf bases form circular scales around the rhizome, which is swollen with food reserve.
Fig : Rhizome of Canna
Fig : Stolon of Imperta Grass
- Stem Tuber
A tuber is a modified stem which develops underground by the swelling of the tip of a stem. Large amount of food materials, primarily in the form of starch, are stored in stem tubers. A typical example is potato with its numerous 'eyes' representing nodes.
In some plants the stem grows horizontally over the surface of the ground. At the nodes of the horizontal stem, adventitious roots develop and lateral buds grow into aerial shoots, so that new plants are formed a short way from the parent. This process gives rise to a series of plants, linked by a horizontal stem called a runner. The runners of certain grasses enable them to colonise bare soil and thus create a lawn.
Fig : Runner of Desmodium triflorumSucker
A sucker is similar to the runner but originates from the basal and underground portion of the main stem. Suckers are generally shorter and stouter, compared to runners.
A slender lateral branch that arises from the base of the main axis is called a stolon. Stolons grow aerially for some time and then arch downwards to touch the ground where its terminal bud gives rise to a new shoot and roots. A common example is Mentha. Runners, suckers, stolons etc., come under the category of creepers.
Fig : MenthaAerial Modifications
Stems show many other modifications to perform certain unusual functions. Stem tendrils are slender, spirally coiled structures that help plants (Cucumber, pumpkin, water-melon, grape-vine etc.,) to climb. These structures develop from axillary buds. In certain cases axillary buds give rise to structures called thorns. These are found in many plants such as Citrus and Bougainvillea. The primary function of thorns is to protect the plants from browsing animals. In cacti and many other plants of the arid regions stems show various modifications. In these plants the stems contain chlorophyll and they conduct photosynthesis. These specialised stems with several nodes and internodes are called phylloclades. Their true leaves are reduced to spines or scales. A cladode is a phylloclade with one or two internodes, resembling a leaf.
Fig : Opuntia