- Books Name
- ACME SMART COACHING Biology Book
- ACME SMART PUBLICATION
- CBSE Class 11
Stem is ascending part of plant and formed by the prolongation of the plumule of embryo.
It is positively phototropic and negatively geotropic and hydrotropic.
It bears nodes and internodes.
Leaf bearing part of stem is called shoot.
It has buds.
It may bear multicellular hair on external surface.
Lateral branches arise from the cortex (exogenous origin).
Bud is a condensed, immature or embryonic shoot with closely placed nodes.
These have a growing point surrounded by closely arranged immature leaves.
Cabbage is the largest bud.
Buds can be Vegetative, Floral and Modified
1. Vegetative buds.
These buds develop into vegetative shoots. They can be:
(a) Terminal or apical bud - Present on the tip of branches.
(b) Axillary or lateral bud - Present in the axil of leaves.
(c) Some plants regularly produce some extra buds on the side of axillary buds called as accessory or supernumerary buds.
(d) Buds which develop from any part of the plant body other than the above mentioned ones are called adventitious buds. These can be :
(i) Cauline buds - Arise directly from stem e.g., Artocarpus (Jack fruit).
(ii) Radical buds - Arise on roots e.g., Sweet potato, Dalbergia.
(iii) Foliar buds - Buds which develop on the leaves e.g., Bryophyllum, Begonia (Elephant ear plant), Kalanchoe etc.
Adventitious buds: A. Radical buds of sweet potato B. Foliar buds of Bryophyllum ,
C. Foliar buds of Begonia D. Bulbil of Dioscorea
2. Floral buds : These buds always develop into flowers.
3. Modified buds: They can be both vegetative or floral buds.
a. Vegetative bud modification
(i) Tendrlis – e.g., Passiflora (Passion flower).
(ii) Thorns – e.g., Citrus (Lemon) , Duranta, Carissa
(iii) Bulbils – e.g., Dioscorea (Yam). Cycas (gymnosperm).
b. Floral bud modifications
(i) Tendrils – e.g., Cardiospermum (Balloon vine).
(ii) Bulbils – e.g., Allium sativum (Garlic)
Types and Modifications of Stem
A. Aerial stems (Epiterranean stem)
It may be reduced, erect and weak.
1. Reduced -Stem reduced to a disc. e.g ., Radish, Carrot, Turnip.
2. Erect stem -It is strong and upright e.g., Maize, Wheat, Mango. An erect stem with swollen nodes is called culm (e.g., bamboos).
3. Weak stems -These are thin, soft and weak and need support. They can be upright or prostrate.
(a) Creepers. The stem creeps on earth and the roots arise at the nodes, e.g., grasses, strawberry, Oxalis.
(b) Trailers – The stem creeps on the ground, but the roots do not arise at the nodes. They may be:
(i) Prostrate or procumbent. The stem creeps on ground totally, e.g., Evolvulus.
(ii) Decumbent. When prostrate stem projects its tip, e.g., Portulaca, Lindenbergia.
(c) Lianas (Stem climber). Woody perennial climbers found in tropical rain forests are lianas. They twine themselves around tall trees to secure sunlight, e.g., Hiptage, Bauhinia vahlii (Phanera).
(d) Climbers. Plants are with long weak stem and have organs of attachment to climb the object. They maybe
(i) Rootlet climbers. Roots produced at nodes help in climbing e.g., Tecoma, Pothos, Piper betel (pan).
(ii) Hook climbers. In Bougainvillea, Ouranta and Carrisa, the thorn is modification of axillary vegetative bud which helps in climbing. In Bignonia, terminal leaflet is converted into hook. Artobotrys and Uncaria are also hook climbers.
(iii) Tendril climbers. Tendrils are thread like structures which help the plants in climbing.
Tendrils are modifications of:
• Entire leaf -Leaf tendril e.g., Lathyrus sativus.
• Leaflet -Leaflet tendril e.g., Pisum.
• Petiole -Petiolar tendril e.g., Clematis, Nepenthes.
• Stipule -Stipular tendril e.g., Smilax.
• Leaf apex -Leaf apex or tip tendril e.g., Gloriosa.
• Inflorescence -Inflorescence tendril e.g., Antigonon.
• Stem -Stem tendril e.g., Vitis (modified apical bud), Passiflora (modified axillary bud).
(e) Twiners. The stem body twines around the support without any special organ of attachment. e.g., Cuscuta, Dolichos and Quisqualis.
B. Underground Stem Modifications
It grows parallel or horizontal to soil surface.
It bears nodes, internodes, buds and scaly leaves e.g., Ginger, Banana, Turmeric, Ferns.
It is of two types:
It is upright or oblique with the tip almost reaching the soil surface e.g.,Dryopteris.
It is horizontal and branched.
Branching may be –
Racemose - Axis is monopodial, e.g., Saccharum, Lotus.
Uniparous cymose - Axis is sympodial, e.g., Zingiber officinale (ginger), Curcuma domestica (turmeric) and Canna.
It is terminal portion of underground stem branch which is swollen on account of accumulation of food, e.g., Potato, Helianthus tuberosus (Jerusalem artichoke).
It grows vertically beneath soil surface.
It is usually unbranched.
It bears nodes, internodes, buds and scale leaves, e.g., Colocasia, Gladiolus, Colchicum, Crocus, Amorphophallus.
Stem is reduced and disc shaped.
The bud is surrounded by many concentric scale leaves.
Leaf bases of inner ones are fleshy and edible and of outer ones are dry e.g., onion, lily, garlic.
It is of two types -tunicated and scaly.
Tunicated bulb is covered by a sheath of membranous scales called tunic.
It may be simple tunicated bulb -covered by a sheath e.g. onion and Narcissus; or compound tunicated bulb-concentric rings of bulblets surrounded by a white membranous sheath or tunic e.g. garlic.
Scaly or naked bulbs do not have tunic. e.g., lily.
Underground modifications of stem: A. Tuber of potato; B. Corm of Colocasia
C, D. Tunicated bulbs of onion (C, entire; D, longitudinally cut) E. Rhizome of ginger
C. Sub-aerial Weak Stem
It is elongated, prostrate, aerial branch with long internodes and roots at nodes
e.g., Oxalis, grasses, Hydrocotyle.
It arises by axillary bud of underground part of stem.
This lateral branch creeps below the soil surface and grows obilquely upward and produces new shoot.
e.g., Banana, Pin epple, Chrysanthemum, rose.
Short horizontal branch producing a cluster of leaves above and the cluster of roots below.
e.g., Pistia, Eichhornia.
It is subterranean long lateral branch arising from base of the stem.
e.g., Colocasia, Rubus, Fragaria
It first grows obliquely upward and then bends down to the ground surface.
Subaerial modifications of stem: A. Runner of Oxalis; B. Offset of Pistia;
C. Sucker of Chrysanthemum; D. Stolon of Fragaria
D. Special Stem Modifications
(a) Phylloclade. It is green flattened or rounded succulent stem with leaves either feebly developed or modified into spines e.g., Opuntia, Casuarina, Muehlenbeckia.
(b) Thorn. It is modification of axillary bud, e.g., Bougainvillea, Duranta, Carissa. Thorns of Alhagi possess flowers, while thorns of Duranta bears leaves.
(c) Cladode. Phylloclade usually having one internode long, is called cladode, e.g., Asparagus, Ruscus. It is of limited growth.
(d) Stem Tendril. It is a leafless, spirally coiled structure found in climbers. It may be a modification of axillary bud, e.g., Passiflora or terminal bud e.g., Vitis.
(e) Bulbils. A condensed, axillary fleshy bud is called bulbil. It helps in vegetative reproduction. e.g., Dioscorea, Globba, Agave, Oxalis.
Functions of a Stem :
1. Mechanical support
These two are normal functions of any stem . Some special functions performed by stem are
3. Food Storage
4. Water Storage
A stem is one of a vascular plant's main structural axes. It provides support for leaves, flowers, and fruits, distributes the water and dissolved chemicals between the roots and the shoots in the xylem and phloem, stores nutrients, and generates new living tissue. The stem is the section of the axis that ascends and bears branches, leaves, flowers, and fruits. It grows from the plumule of a sprouting seed's embryo. Nodes and internodes are found on the stem. Nodes are the parts of the stem where leaves are born, whereas internodes are the parts between two nodes. The stem produces buds that are either terminal or axillary. When young, the stem is usually green, but as it matures, it becomes woody and dark brown.
The stem's primary job is to spread out branches containing leaves, flowers, and fruits. Water, minerals, and photosynthates are all carried by it. Some stems have multiple functions, including food storage, support, protection, and vegetative propagation.
Modifications of a Stem: Stems havebeen altered to do various tasks. In potato, ginger, turmeric, zaminkand, and Colocasia, underground stems have been adapted to store food. They also serve as perennation organs, allowing them to survive in environments that are not conducive to growth. Stem tendrils, which emerge from axillary buds and are slender and spirally coiled, assist plants in climbing, such as cucumbers, pumpkins, and melons. Stem axillary buds can also be transformed into woody, straight, and pointed thorns.
Many plants, such as Citrus and Bougainvillea, have thorns. They keep animals from eating the plants. Arid-climate plants change their stems into flattened (Opuntia) or fleshy cylindrical (Euphorbia) forms.They have chlorophyll and are capable of photosynthesis.
Some plants, such as grass and strawberry, stretch underground stems to new niches, and new plants emerge as older sections die. A slender lateral branch emerges from the base of the main axis in plants like mint and jasmine, and after growing aerially for a time, arch downwards to touch the ground.
Aquatic plants like Pistia and Eichhornia have a lateral branch with short internodes and each node containing a rosette of leaves and a tuft of roots. The lateral branches of bananas, pineapples, and Chrysanthemums begin from the main stem's basal and underground portion, develop horizontally beneath the soil, and then emerge obliquely upward, giving rise to leafy shoots.
Figure 6(b): Stem modifications