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Morphology is concerned with external characteristics while anatomy is concerned with internal characteristics. Stem is negatively geotropic and positively phototropic i.e. it grows away from the earth and towards light. Young stems are green in colour and help in photosynthesis. Mature stems are generally brown and they have nodes and internodes. From the nodes axillary buds develop which then becomes a new stem

There are three types of buds-

1. Terminal (apical)
2. Axillary
3. Auxillary

Stem

Axillary is present on the axil of the leaf i.e. between the stem and the leaf. From the axillary buds branches arise. Sometimes from axillary buds instead of branches flower also arise then these buds are called floral buds. The distance between two adjacent nodes is called internode. The slight swelling from where the axillary buds and leaves originate is called node. If nodes are present clearly they cam be easily identified. In mature stems nodes are not easily identified. Stems support the remaining part of the shoot. It transports water and minerals from roots to leaves and prepared food from leaves to all parts of the plant.

Plants will grow internally but they are not able to grow externally at the same time. Hence cracks are developed as it cannot hold the structure. This happens in dicot plants because in monocot plants there is no external growth. Through these gaps exchange of gases will take place. They are called lenticels.

Stomata are mall pores present on the surface of leaves and stems (immature). They are guarded by guard cells which contain chlorophyll. Stomata opens during the day and closes during the night by the help of guard cells. Branches can be differentiated into nodes and internodes because of leaves. Chloroplasts identify the presence of light. Stem ,roots and leaves are the vegetative parts of the plant. Stem is modified for additional functions also.

Stem is modified for food storage.

Tuber -. There are some underground stem which swell up and the swell up structure is called stem tuber. If we analyze it then we find that it contains several eyes. Scale leaves and axillary buds together form the eyes of the potato. Eyes are protected by scale leaves eyes can be used as a means of vegetative reproduction. Seed are not used as they have less food and those plants which grow are weak. To provide with strong plants we use eyes. Depressions are nodes e.g. potato.

Rhizome - it grows horizontally and underground to the surface of soil. It has regular nodes and internodes. The nodes and internodes are protected by scale leaves. In the axil of the scale leaves axillary buds are present e.g. ginger.

Corm - it grows vertically to the surface of the soil i.e. downwards. They are swollen up structures axillary buds are present but they are not arranged properly i.e. internodes are present but not identified. Scale leaves are there around axillary bud. Contractile roots are present and they fix into the ground so that the stem will grow downwards. Axillary buds develop into a new corm. e.g. colocassia.

Bulb- here food is stored in scale leaves. Leucoplasts store food. Terminal bud or axillary bud develops into new aerial shoot. e.g. onion.

Sub aerial stem modifications

Here stem is on the surface of the soil and performs additional function i.e. vegetative propagation.

Runner - branches originate from the main plant in all directions. From parent plant daughter plants are produced. After growing for some time with the mother plant the daughter plant is separated from the mother plant. In most plants vegetative propagation takes place. Only the mother plants contain true roots the daughter plants contain adventitious roots.

Stolon - in comparison to the runner the stems are thick. They originate from the base of the mother plant. It grows into the air for some time and it finally reaches the ground.
Offset - it is generally in aquatic plants. It is a short and thick stem.

Sucker - they originate from underground and grow obliquely and reach above the soil. When it comes up a new plant is formed.
Aerial stem modification
They grow above the soil and perform additional function and provide support to the plant.

Stem tendril - in some weak plants the stem modifies and forms a coiled wiry structure and attaches itself to a hard object. It is used for climbing on hard objects for support. If the tendril originates from axillary or terminal bud then it is called stem tendril. If it is growing from the tip it is called terminal bud. If it is growing from axil it is called axillary bud.

Twiner - the stem itself turns around the solid object. There is no thorn or tendril.

Thorns - it is present for protection from the enemies. Axillary buds are modified into thorns. Thorns are hard in nature whereas spines are comparatively soft. Normally spines are developed from the leaves.

Spines are also known as prickles. If the thorn or the originates from the axillary or terminal bud then it is called a stem modification. If it is present just outside the branch then it is called leaf modification. If it is present on any other part other than the ones mentioned above then it is called epidermal hairs.

Phyloclade - if the stem does photosynthesis then it is called a phyloclade. It is flat, green and thick. It is flat for increasing the surface area, thick for storing water and green for photosynthesis. In this leaves are modified into spines to reduce transpiration (evaporation of water from aerial parts of the plant). Cuticle is an outer covering which is present for preventing transpiration.

Cladode - stem is modified for photosynthesis. Therefore it is green in color but thick and short of only one internode length. Axillary buds are modified into thin ,short and greenish structures. These branch like structures are called cladode. Leaves are modified into spines. The function of spines is protection as well as to reduce transpiration.

Bulbil - some auxillary buds or adventitious buds develop from the plant and store food material and they detach from the plant and develop into new plants.

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