Definition of Stem:
The aerial part of the plant body is collectively described as shoot system. Main axis of this shoot system is called stem. The stem is main ascending axis of the plant i.e. positively Phototropic.
Functions of the stem:
1) They are to produce and support lateral appendages such as branches, leaves, flowers, and fruits.
2) It conducts water and minerals to different parts of the shoot and transportation of food to all plant parts.
Modifications of stem :
To perform special functions other than Or in addition to the normal functions, stem shows various modifications in shape, form, and structure. Some of which are described as :
1) Underground modification.
2) Sub-aerial modifications.
3) Aerial modifications.
Types of stem modification:
1) Underground modification:
In many herbaceous plants, stem develops below the soil and is called underground stem. The stem varies in shape and size according to the amount of accumulated food.
They are distinguished from the storage roots by the presence of the nodes, internodes, scaly leaves or buds. The underground modifications are of following types.
a) Rhizome:
It is modified underground stem which is thick and elongated. It usually runs horizontally. It is distinguished by the nodes, internodes, scaly leaves, bud and adventitious roots.
Examples: Ginger, Canna etc.
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| Rhizome |
b) Stem Tuber:
Tubers are usually the swollen tips or ends of special underground branches swollen due to storage of food (carbohydrate like starch).
The tubers shows nodes and internodes and nodes bear scale leaves with axillary buds, commonly called 'eye's under favourable conditions 'eye's sprout and produce aerial shoots. Thus tubers helps in vegetative propagation. Tubers do not produce adventitious roots, thus they differ from rhizomes.
Example: Potato.
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| Stem tuber |
c) Bulb:
It is an underground, pyriform or spherical modified shoot having highly reduced disc-shaped stem and several fleshy scale leaves growing from it and enclosing a terminal bud.
From the latter develops a leafless floral axis called scape. The leaf bases bear axillary buds. The adventitious roots are produced from the lower side of the reduced stem. The bulbs are of two types:
1) Tunicated bulb:
When the fleshy leaves surround the apical bud in the form of concentric rings, it is called tunicated bulb.
Example: Onion.
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| Tunicated bulb |
2) Non-tunicated Bulb:
In scaly bulb, the scale leaves are not arranged in concentric manner. The scale leaves are loose and overlapping. The bulb is not covered by membranous covering.
Example: Garlic.
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| Non-tunicated bulb |
d) Corm:
Corm is short, stout, fleshy, upright and thickened underground stem. It bears many buds in the axils of scale leaves which develop into daughter corms. At the bases or even from side of stem adventitious root develop. Corm is a condensed form of rhizome growing vertically.
Example: Amorphophallus.
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| Corm |
2) Sub-aerial modification:
In this type of modification, stem trails on the soil or is found partially beneath the soil in such way that it remains in contact with both the soil and air.These modifications are of 4 types:
a) Runner:
These are special narrow, green, horizontal or prostrate branches which develop at the base of erect shoots called crowns.A number of runners arise from each erect shoot. They spread in different directions and bear new crowns above and tufts of adventitious root below at certain intervals. Each runner one or more nodes. The nodes bear scale leaves and axillary buds.
Example: Cynodon (Lawn grass), Oxalis etc.
Cynodon
b) Stolon:
It is a special branch which grows upwards in the beginning but then bends down to the soil to produce a new plant like runner; stolon is responsible for Vegetative reproduction.Examples: Jasmine, Mentha, Nephrolepis.
Jasmine
c) Offset:
It is commonly called the runner of aquatic plants. It is shorter and thicker than runner. It helps in the vegetative propagation in aquatic plants.Example:
Eichhornia, Pistia.
Pistia
d) Sucker:
Sucker is a runner like non-green branch which develops from the axil of scale leaf in the underground part of stem. It grows horizontally below the soil for some distance and comes above the soil obliquely and produces green leaves to form aerial shoot. The sucker can, therefore be called underground runner.Example: Chrysanthemum, Mentha (Mint).
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| Mentha |
3) Aerial modifications :
some aerial stem or its vegetative or floral buds undergo to form specialized structures like tendrils, thorns, phylloclades or bulbils to perform specific function. Following are the types of aerial modification of stem.
a) Stem tendril:
It is modification of stem in which axillary bud modifies to form a thin wiry, and highly sensitive structure called tendril. Tendril help the plant to attach itself to the support and climb.
Example: Cucurbita, Passiflora (Passion flower).
Cucurbita
b) Phylloclade:
The phylloclade is a stem which gets transformed into like structure. The phylloclade is green, flattened structure and distinct nodes and internodes. It is thick, fleshy and succulent.
Example: Opuntia.
Opunita
c) Cladode:
These are green branches of limited growth which have taken up the function of photosynthesis. True leaves are reduced to scales or spines.
Example: Asparagus.
Asparagus
5) Bulbils:
When axillary bud becomes fleshy and rounded due to storage of food, it is called bulbil. It gets detached from the plant, falls on ground and develops into a new plant.
Example: Dioscorea.
Dioscorea
So, this are the all about what is Stem modification? Types of stem modification etc.