VEGETATIVE PROPAGATION

          

Vegetative propagation is a form of asexual reproduction of a plant. Only one plant is involved and the offspring is the result of one parent. The new plant is genetically identical to the parent. 

NATURAL VEGETATIVE PROPAGATION

New plants grow from parts of the parent plant. They include:

• Stems: Runners are stems that grow horizontally above the ground. They have nodes where buds are formed. These buds grow into a new plant.

• Roots: New plants will grow out of swollen, modified roots called tubers. Buds develop at the base of the stem and then grow into new plants.

• Leaves: Leaves of some plants will grow into a new plant if they become detached from the parent plant. Other plants grow small plants called plantlets on the edge of their leaves.

• Bulbs: A bulb contains an underground stem. Leaves are attached to the stem. These leaves contain much stored food. At the centre of the bulb is an apical bud. Also attached are lateral buds. The apical bud will produce leaves and a flower while the lateral buds will produce new shoots. As the plant grows and develops it will form a new bulb underground.

Artificial Vegetative Reproduction

      Horticulturists and farmers use artificial means to produce plants that are identical to the parent plant. Some of the methods used are:

·         Cuttings: Cuttings are part of the plant that is cut off of the parent plant. Shoots with leaves attached are usually used. New roots and leaves will grow from the cutting. The shoot is cut at an angle. A growth promoter may be used to help with the growth of the roots. 

·        Grafting: In grafting 2 plants are used to develop a new plant with combined traits from the 2 parent plants. In grafting the scion is the above ground part of one plant. The scion is attached to the stock which is the rooted part of the second plant. 

·        Layering: In layering a shoot of a parent plant is bent until it can be covered by soil. The tip of the shoot remains above ground. New roots and eventually a new plant will grow. These plants can then be separated.

Larynx

Function of the Larynx

The larynx is situated at the upper end of the trachea, connecting the lungs to the vocal tract. It has two main functions:

• The first, and most important with respect to maintaining one’s health status, is as a valve. Thus, during the act of swallowing the larynx is closed, thereby ensuring that food passes safely into the esophagus and down into the stomach, rather than entering the lungs.

• The second function, and more important to the theme of this website, is as a source of sound to be used in speech.

Laryngeal Cartilages

The larynx is constructed from nine cartilages. For the purpose of this discussion, however, we will restrict ourselves to a consideration of just three of these:

• thyroid cartilage

• cricoid cartilage

• arytenoid cartilages

The thyroid cartilage is the largest of the laryngeal cartilages and forms the major chamber of the larynx. It is constructed from two relatively flat quadrilateral (four-sided) plates that are fused anteriorly in the midline along most of their length into a characteristic shield shape. The angle at which the plates join is different in males and females. In males it is about 90^o and in females it is nearer 120^o. This so-called thyroid angle is, therefore, more acute in men and it is consequently more noticeable as the Adam’s Apple. Posteriorly each plate is extended both upwards and downwards into horns or cornu. The superior horns attach by ligaments to the hyoid bone, situated under the base of the tongue in the neck, and the inferior horns articulate with the cricoid cartilage beneath.

The cricoid cartilage, therefore, forms the base of the larynx. It is a complete ring of cartilage that extends upwards posteriorly to form a plate. The inferior horns of the thyroid cartilage sit on the cricoid cartilage at this posterior position. This articulation allows the cricoid cartilage to be tilted by an angle of up to 15^o downwards and away from the lower anterior border of the thyroid cartilage. The cricoid cartilage is connected underneath to the upper trachea by the crico-tracheal ligament. We see, therefore, that both the thyroid cartilage and cricoid cartilages are individual cartilages (see Figure 1).

Figure 1. Human Larynx

The third major cartilage construction in the larynx is actually a pair of cartilages known as the arytenoid cartilages. These are pyramid-shaped and they are situated on the upper posterior part of the cricoid cartilage. One is placed laterally to the left and one laterally to the right. The arytenoids can be moved in rotational and sliding movements that are used to control the movement of the attached vocal cords.

Breathing Mechanism

The physical movements associated with the gaseous exchange are called breathing. They are controlled by the respiratory centre of the medulla oblongata in the human brain. Thus, the breathing movements are involuntary to a large extent. However, we can control the rate of breathing and the extent of breathing but not for a long time. The respiratory centre is stimulated by the carbon dioxide concentration of the blood.
There are two types of physical movements associated with the gaseous exchange. They are:

• Inspiration or inhalation

• Expiration or exhalation

Inspiration or Inhalation

During inspiration, the outer intercostal muscles contract, which raises the chest cavity or the ribs. This is accompanied by the lowering of the diaphragm. Together these movements serve to increase the area of the thoracic cavity, which reduces the pressure. The air from outside rushes into the lungs.

Inspiration After the internal respiration in the lungs, the impure air is expelled in the following manner:

Expiration

Expiration or Exhalation

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The inner intercostal muscles contract bringing the ribs back to the original position and the diaphragm is also raised back by the action of the abdominal muscles. This reduces the space in the chest cavity and increases the pressure. This expels the air out of the lungs.

Demonstration of Breathing Mechanism

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Take a bell jar. Towards its rounded end, fix a 'Y' shaped glass tube and on the open ends of the two branches tie a balloon each. On its open end tie a thin rubber sheet. The cavity of the bell jar acts as the thoracic cavity, the "Y" shaped tube as the trachea that branches into bronchi and the rubber sheet as the diaphragm.

Experiment Demonstrating Inspiration To demonstrate inspiration, pull the rubber sheet down. The balloons get inflated. This shows how the lungs are filled with air when the diaphragm moves down.

How are the Balloons Inflated?

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When the rubber sheet is pulled down, the volume of the cavity increases. This brings the pressure down. Low pressure inside the jar brings the air rushing in through the tubes and inflates the balloon.

Experiment Demonstrating Expiration To demonstrate expiration, the rubber sheet is allowed to go back to its original position. The balloons get deflated. This shows how the lungs are deflated when the diaphragm comes back to its original position.