Types of Stem in Plant :
Stems are generally rigid, firm and strong which enable the plant to remain in an upright position, but there are plants having weak stem and which are not able to grow in an upright position.
Aerial stems may be (a) strong (erect) or (b) weak.
A. STRONG STEM:- Plants become erect and remain in an upright position due to the rigidity of the stem, as seen in herbs, shrubs and trees.
Types of strong plant stem :
(a) Excurrent:- When the tree takes a pyramidal form due to the development of racemose branching it is known as excurrent e.g. species of Abies, Picea, Pinus (Gymnosperms), Polyalthia longifolia (Annonaceae) etc.
(b) Deliquescent:- When the tree takes a dome-shaped (more or less rounded) appearance due to cymose branching then it is known as deliquescent, e.g. Mangifera indica (Anacardiaceae).
(c) Caudex:- Tall erect unbranched stem is known as caudex. The stem bears leaves at its apex and annular scars of fallen leaf bases, e.g. Cocos nucifera (Palmae), Borassus flabellifer (Palmae) etc.
(d) Culm:- Jointed stems are known as culm. In this case the nodes are solid and the internodes are hollow, e.g. grasses such as Bambusa arundinacea (Gramineae).
Scape:- In some monocotyledonous plants the suppressed underground stem produces an erect unbranched aerial shoot which is known as scape. It comes out through the cluster of leaves and bears at its apex either a solitary flower or a cluster of flowers. The plant is defined as acaulescent, e.g. Polyanthes tuberosa (Amaryllidaceae), Allium cepa (Liliaceae), aroids, etc.
B. WEAK STEMS:- Many stems are weak, they cannot maintain an upright position.
Types of weak plant stem:— Weak stems are either (a) Trailer — trails over the surface of the ground without rooting at the nodes, or (b) Creeper — grows horizontally over the surface of the soil striking roots (arising from the nodes) at short distance or (c) Climber — climbs up by supporting any other plant or object.
(a) Trailer or Trailing plants :
(i) Procumbent or Prostrate — When the stem lies prostrate over the ground, its apex also lies flat over the surface e.g. Basella rubra (Chenopodiaceae), Evolvulus nummularius, Ipomoea reptans (Convolvulaceae) etc.
(ii) Decumbent — When the stem lies over the ground with the apex turned upwards e.g. Tridax procumbens (Compositae), Lindenbergia indica (Scrophulariaceae) etc.
(b) Creepers – When the plant grows horizontally over the surface of the soil and produces branches profusely and spreads out in all directions and gets rooted at each nodes, it is known as a creeper, e.g. Ipomoea batatas (Convolvulaceae), Cynodon dactylon (Gramineae), Phyla nodiflora (Verbenaceae) etc.
(c) Climbers or Climbing plants :
1. Stem climbers or Twiners :- Weak, long and slender stems of some plants like Clitorea ternatea (Leguminosae-Papilionaceae), Dioscorea alata (Dioscoreaceae), Quisqualis sp. (Combretaceae), Ipomoea quamoclit (Convolvulaceae) etc. climb up other plants or objects by twining round their own stem.
Twiners are classified into two groups according to the direction of twining –
(i) Dextrorse – When the climbers twine clockwise or to the right then it is called dextrorse e.g. Dioscorea alata (Dioscoreaceae), Mikania scandens (Compositae), Phaseolus sp. (Leguminosae-Papilionaceae), Dolichos lablab (Papilionaceae) etc.
(ii) Sinistrorse – When the climbers twine in anticlockwise or to the left then it is called sinistrorse e.g. Dioscorea bulbifera (Dioscoreaceae), Clitorea ternatea (Leguminosae-Papilionaceae), Ipomoea quamoclit (Convolvulaceae), Convolvulus sp. (Convolvulaceae) etc.
2. Lianes — Long and woody perennial stem climbers which climb up tall forest trees are called lianes. They climb up owing to the different growth and curvature of the stem e.g. Hiptage benghalensis (Malpighiaceae), Beaumontia grandiflora (Apocynaceae), Marsdenia (=Dregea) volubilis (Asclepiadaceae), Bauhinia vahlii (Leguminosae-Caesalpinaceae) etc.
3. Tendril climbers — There are some plants which develop special type of climbing organs called tendrils. These are slender, spirally coiled, thread-like structures which may be modifications of either branches or leaves or inflorescence stalk and are very sensitive to contact.
They help the plant to climb up any other plant or object. Different plant organs are modified to form tendrils.
(i) Tendrils may develop from the axil of leaf and are called stem tendrils; either the axillary bud is modified into slender coily structure e.g. Passiflora sp. (Passifloraceae), or as in Vitis sp. (Vitaceae) the apical bud is modified into tendril.
(ii) Entire leaf is modified into tendril e.g. Lathyrus aphaca (Leguminosae-Papilionaceae).
(iii) Terminal leaflets are modified into tendrils e.g. Pisum sativum (Leguminosae-Papilionaceae).
(iv) Apex of the leaf is modified into tendril e.g. Gloriosa superba (Liliaceae).
(v) Stipules are modified into tendrils e.g. Smilax sp. (Liliaceae).
(vi) Inflorescence axis is modified into tendrils e.g. Antigonon leptopus (Polygonaceae), Cardiospermum helicacabum (Sapindaceae) etc.
4. Root climbers: – Several weak plants like Pothos scandens (Araceae), Piper betle (Piperaceae), Scindapsus officinalis (Araceae), Ficus pumila (Moraceae) etc. climb up suitable objects with the help of adventitious roots which develop from the nodes of the stem.
5. Hook climbers :- In Artabotrys uncinatus (Anonaceae) curved hooks are developed from the flower stalks (pedicels) which help the plant to climb up any other plant or support.
Hook-like structures are also formed due to the modification of terminal leaflets which also serve the climbing purpose e.g. Bignonia unguis-cati (Bignoniaceae); sometimes hooks are modified inflorescence axes, e.g. Artabotrys sp. (Anonaceae).
6. Leaf Climbers:- Leaf climbers are the climbing plants in which the leaf or part of the leaf is modified into tendril and which act as climbing organ (see iv & v under tendril climbers).
The leaf-stalk i.e. petiole of Clematis sp. (Ranunculaceae) is sensitive to contact and coil round any neighbouring object helping the plant to climb. In Nepenthes sp. (Nepenthaceae) the modified petiole often twists round the support like tendril holding the pitcher in vertical position.
7. Rambler or Scramblers:- Several plants have been found to climb neighbouring plants in forests with the help of prickles and thorns e.g. Bougainvillea spectabilis (Nyctaginaceae), climbing rose, Calamus rotang (Palmae) etc.
In Calamus rotang, long slender whip-like stalk covered by prickles (hook-like) is produced from the leaf sheath. This helps the plant to climb neighbouring plants or objects.
8. There is another type of climber called adhesive climber. These climbers are provided with adhesive discs to some of their organs, by means of such adhesive discs they may adhere to flat surfaces, e.g. Ampelopsis veitchii (Vitaceae). In this plant the adhesive discs are developed from the tendrils.
Functions of Stem in Plant :
I. NORMAL FUNCTIONS OF STEM IN PLANT
(a) Mechanical function is the bearing of the crown and weight of entire plant, the production and bearing of foliage leaves, branches and reproductive structures like flowers and fruits.
(b) Physiological function – Conduction of mineral salts and water absorbed by roots and translocation of prepared food matters to various plant parts.
II. SPECIAL FUNCTIONS OF STEM IN PLANT
(a) Storage of water – This is done by special water storage tissue present beneath the epidermis of stem in plant, as in many Cactaceae ; the best example of storing much quantity of water in the stem is afforded by the species of Echinocactus (Cactaceae) of Mexico.
(b) Storage of food (mainly carbohydrates), this is done mainly by underground modified stems like rhizomes, tubers and corms of many plants. Sometimes the aerial stems store up starch e.g. stem of Amaranthus tricolor (Amaranthaceae).
(c) Photosynthetic function i.e. manufacture of carbohydrate food is carried out by most green stems, particularly by modified leaf-like branches or phylloclades and cladodes of Muehlenbeckia sp. (Polygonaceae) and Ruscus aculeatus (Liliaceae).
(d) Self defence i.e. by the development of thorns and prickles-thorny and prickly stems are meant for defence against grazing animals. Examples of thorns are afforded by the species of Vangueria (Rubiaceae), Duranta (Verbenaceae), Alangium (Alangiaceae etc., prickles occur in stems of Rosa sp.
(e) Stems and branches as supporting organs — similar to the function of stems and branches as climbing organs, this is done by branch tendrils of the species of Vitis (Vitaceae), Passiflora (Passifloraceae) etc.
(f) Propagation of vegetative parts — Sub-aerial modified stems like runner, stolon, sucker and offset help in vegetative propagation and perennation.
Modification of Stem in Plant :
In some cases instead of growing upwards, the stem may grow underground. The stem undergoes modifications or metamorphosis and assumes different shapes to perform various functions other than the normal ones.
Functions of Underground Stem Modification :
(i) Perennation:- Survival through unfavourable conditions for many seasons.
(ii) Vegetative propagation:- New plants are formed from the vegetative parts.
(iii) Storage of food and water:- Food matters and water are stored within the underground stems.
Character of the Underground Stems differing from Roots :
(1) Presence of nodes and internodes.
(2) Presence of small dry, scale leaves and development of adventitious roots from the nodes.
(3) Presence of buds at the axils of scale leaves.
(4) Internal structures are like those of stems.
Types of Modified Underground Stem with Examples :
1. RHIZOME:- It is a thick, fleshy underground plant stem with distinct nodes and internoces, growing horizontally, sometimes more or less vertically beneath the surface of the soil.
The presence of distinct brown scale leaves at the nodes with the axillary buds in most cases indicates the nature of the stem. From the lower surface of the nodes, a large number of adventitious roots are developed.
Under favourable condition the terminal bud develops into aerial shoot. At the close of the vegetative season the annual parts die, leaving a scar on the upper surface of the rhizomes. The rhizome continues its growth by one or more lateral buds, thereby it grows year after year.
It is found in herbaceous perennial monocotyledons e.g., Zingiber officinale (Zingiberaceae), Musa paradisiaca (Musaceae), Curcuma domestica (Zingiberaceae) etc.
In some plants like Alocasia indica (Araceae), Nymphaea esculenta (Water-lily) of Nymphaeaceae, the rhizome grows in vertical direction instead of being horizontal. Those stout vertical rhizomes are called root-stock.
When the rhizome is wiry, horizontal and has long internodes, it is called a sobole. Common examples are some members of the family Gramineae viz., species of Agropyron, Spinifex littoreus, Cynodon dactylon etc.
Some botanists consider sobole as underground runner or creeping stem due to thin and wiry nature of the stem devoid of accumulated food matter.
2. STEM TUBER:- It is the swollen tip of an underground creeping branch which arises from the axil of a leaf on the main stem. Its further growth in length is retarded, the tip becomes swollen and enlarged owing to the deposit of food matters.
The shape of the tuber is round or oval and it possess several irregular internodes. On the surface of the tuber eyes are present.
Scars of minute scale leaves which fall off very early are present-these bear normal buds in their axils. Adventitious buds are absent in the tuber. Examples of tuber are Solanum tuberosum (Potato – Solanaceae), Cyperus rotundus (Cyperaceae) etc.
3. CORM:- It is an enlarged, more or less round, solid, vertical fleshy underground stem with a single internode with many adventitious buds and roots on the surface of its body. The scale (thin and membranous) leaves are few and often situated at the apex encircling the petiole of the foliage leaf.
Buds are present; some of them develop into new young corms and the old corm ultimately dies. Adventitious roots grow either from all over the body or from the base of the corm, e.g. Amorphophallus campanulatus (Araceae), Crocus sativus (Liliaceae) etc.
4. BULB:- The bulb is a small, modified disc-like underground stem. The stem of plant is extremely reduced and takes the shape of a disc. Thick and fleshy leaf bases are crowded in concentric manner one above the other over the discoid stem which bears numerous fibrous adventitious roots at its base.
Food materials are stored within the fleshy underground scale leaves. Axillary buds are present in the axil of leaf bases and also a terminal bud in the centre of the disc.
There are two kinds of bulbs:-
(a) Tunicated bulb – Here fleshy leaf bases are arranged in a concentric manner and the scale leaves become dry, thin and membranous to form common covering or tunica, hence the bulb is known as tunicated bulb, e.g. Allium cepa (Liliaceae).
(b) Scaly or naked bulb — Here the thick and fleshy leaf bases are not arranged in a concentric manner, they overlap one another and are not covered by tunic (dry scale leaves), e.g. Lilium candidum, Tulipa gesneriana (Liliaceae).
Sub-aerial Modified Stem with Examples:
There are four types of sub-aerial modified stems for vegetative propagation, such as –
(1) RUNNER — It is a slender, prostrate, creeping aerial stem or branch. After running a short distance over the earth in a creeping manner it sends off roots at its end and leaves develop, thus producing a new plant which produces another runner from its leaf-axil which behaves similarly.
Thus a series of independent plants are linked together by several runners. Sometimes runner bears scale leaves, e.g. Centella asiatica (Umbelliferae), Oxalis corniculata (Oxalidaceae), Marsilea sp. (Pteridophyte) etc.
(2) OFFSET:- It is just like runner, but the stem is shorter and thicker. The branch produces at the ends cluster of roots and tuft of leaves e.g. Pistia stratiotes (Araceae), Eichhornia crassipes (Pontederiaceae) etc.
(3) SUCKER: This is a sub-aerial branch arising from the stem below the surface of the earth and which after growing in a horizontal direction for certain distance produces roots and turns upwards above the ground; ultimately it develops into a new plant e.g. Mentha spicata (Labiatae), Chrysanthemum coronarium (Compositae) etc. .
(4) STOLON:— It is almost like a runner but the branch is more slender and longer, and arches due to its great length. On touching the soil the tip produces cluster of roots and leaves, e.g. Fragaria vesca (Rosaceae).
Function of Sub-aerial Modified Stem :
The function of sub-aerial modified stems in many cases is quick vegetative propagation, e.g. species of Pistia and Eichhornia (Water hyacinth).
Metamorphosed Aerial Stem with Examples :
Aerial stems undergo extreme modifications i.e. metamorphosis like all other organs to discharge various functions other than normal. These are as follows:
(a) Stem or Branch tendril:- It is a metamorphosed stem or branch, slender and coiled, meant for the support of weak stems. In many cases, as in Vitis the upper extremity is transformed into a coily slender tendrillar part.
From the axil of leaf on the lower unchanged part another branch comes out whose apical portion is similarly transformed into a tendril and so on; the branches are thus, arranged in scorpioid cyme.
The pseudo axes or branches become straight with the leaves opposite to the tendril. In Passiflora foetida (Passifloraceae), the tendril is originated in the axil of the leaf.
Function: Serves as a climbing organ.
(b) Thorn:- It is an axillary branch. Its further growth is arrested or checked and ultimately converted into a hard, sharp and pointed structure.
Sometimes it bears small leaves, otherwise its axillarry position indicates its branch nature. Examples: Duranta repens (Verbenaceae), Alangium salvifolium (Umbelliferae) etc.
Function – The thorn serves as a defensive organ.
(c) Phylloclades or Cladophylls and Cladodes:- Phylloclade or cladophyll is a flat leaf-like aerial metamorphosed stem or branch of many or several nodes and internodes doing the function of leaves.
In many plants growing in dry or arid regions, leaves are reduced to spines or leaves fall off very early as an adaptation and precaution against loss of water.
The phylloclades function as leaves and sometimes act as water storing organs. Examples — Opuntia dillenii (Cactaceae), Muehlenbeckia platyclados (Polygonaceae) etc. are typical phylloclades with distinct nodes and internodes.
In some plants like Asparagus racemosus (Liliaceae) and Ruscus aculeatus (Liliaceae), each branch consists of one internode arising at the axil of minute scale leaves. The branches become very much leaf-like. These metamorphosed leaf-like branches of one internode are called cladodes. Phylloclades and cladodes do the function of foliage leaves i.e. photosynthetic.
(d) Pseudo-bulb:- It is also a modified stem as seen in aerial orchids. Here generally one internode of the stem becomes modified into a fleshy and tuberous structure.
Its function is to store moisture in excess so that the plant can survive during unfavourable condition.
(e) Bulbils:- These are aerial bulbs arising at the axil of foliage leaves or scale leaves. Sometimes they consist of disc-like stem covered by leaves as in Agave sp.
Bulbils are also present in Lilium bulbiferum (Liliaceae), Globba bulbifera (Zingiberaceae). In Agave cantala (Agavaceae) the bulbils are found to produce new plants while still attached to the stem – hence the bulbils are viviparous.