Plants - Classification Based On Growth Habit


We see a variety of plants around us. Despite the fact, they all have same parts and the same functions. They appear unique with different types of stem, leaves, flowers, seeds etc. Therefore, the classification of plants is mainly based on several factors and it can be further classified based on their height, tenderness of stem, branches and their life cycle.

Plants and Its Classification

If we consider plants, based on their height some are too short while some are too tall to climb. Besides the height, stem thickness, delicacy also varies. For example- Short plants have greenish, soft, weak stems and big tall plants or trees have a thick, strong and woody stem which is hard to break.

Based on growth habit plants are broadly categorized into three groups. They are as follows:

  • Herbs: Starting from the smallest, herb is a short plant with green, delicate stem without the woody tissues. Generally, they have few branches or branchless. These can be easily uprooted from the soil.They contain enough nutritional benefits and vitamins to make it a part of the diet. Tomato, wheat, grass are few examples of herbs.
  • Shrubs: Shrubs are much taller than herbs which might be of our height. Shrub is a medium-sized plant with bushy, hard stems with branches. Although stems are hard, they are flexible but not fragile. Rose, lemon, and henna are some of the common shrubs around us.
  • Trees: Trees are big and tall plants. They have very thick and hard stems called the trunk. This single main stem i.e., trunk give rises to many branches bearing leaves and fruits. Some trees are branchless like coconut tree; they have only one main stem which bears leaves, flowers, and fruits all by itself. Banyan, mango, cashew, are some examples of trees.

Transport of Food in Plants

Food is manufactured in the leaves and green stems of the plant during a process called photosynthesis. The food made by the leaves is in the form of a simple sugar named glucose. Other types of substances are called plant hormones; they are released from the tips of roots and shoots. The movement of food from leaves to other parts of the plant is called translocation. In plants, the phloem vessel translocates the food and other substances.

Phloem consists of sieve tubes and companion cells. Sieve tubes are living cells which contain cytoplasm but do not have a nucleus. So its function is supported by a companion cell. The phloem vessel in the vascular bundle will transport sucrose and amino acids away from the leaf. As the vascular bundles are connected with similar structures in the roots, each sieve tube is continuous with those in the roots. Sucrose and amino acids enters the phloem by active transport. Later, it is translocated from the sources to the sinks.

Food is prepared in the mesophyll cells of the leaf. The palisade mesophyll cell is just below the upper epidermis layer. It that contains more chloroplats than the spongy mesophyll layer. The spongy mesophyll layer is below the palisade mesophyll layers that have large air sacs. There is a slight separation between the cells to provide absorption of carbon dioxide; this separation must be very little to support capillary action for water distribution. Food is translocated in the form of sucrose. The movement of water and dissolved minerals in xylem is always upward, from the soil to the leaves. The movement of food can be upward as well as down ward depending upon the needs of the plants.

Fig. Xylem vessel and Phloem vessel

Human Circulatory System

The human circulatory system functions to transport blood and oxygen from the lungs to the various tissues of the body. The heart pumps the blood throughout the body. The lymphatic system is an extension of the human circulatory system that includes cell-mediated and antibody-mediated immune systems. The components of the human circulatory system include the heart, blood, red and white blood cells, platelets, and the lymphatic system.


The human heart is about the size of a clenched fist. It contains four chambers: two atria and two ventricles. Oxygen-poor blood enters the right atrium through a major vein called the vena cava. The blood passes through the tricuspid valve into the right ventricle. Next, the blood is pumped through the pulmonary artery to the lungs for gas exchange. Oxygen-rich blood returns to the left atrium via the pulmonary vein. The oxygen-rich blood flows through the bicuspid (mitral) valve into the left ventricle, from which it is pumped through a major artery, the aorta. Two valves called semi lunar valves are found in the pulmonary artery and aorta.

The ventricles contract about 70 times per minute, which represents a person's pulse rate. Blood pressure, in contrast, is the pressure exerted against the walls of the arteries. Blood pressure is measured by noting the height to which a column of mercury can be pushed by the blood pressing against the arterial walls. A normal blood pressure is a height of 120 millimeters of mercury during heart contraction (systole) and a height of 80 millimeters of mercury during heart relaxation (diastole). Normal blood pressure is usually expressed as "120 over 80."


Blood is the medium of transport in the body. The fluid portion of the blood, the plasma, is a straw-colored liquid composed primarily of water. All the important nutrients, the hormones, and the clotting proteins, as well as the waste products, are transported in the plasma. Red blood cells and white blood cells are also suspended in the plasma. Plasma from which the clotting proteins have been removed is called serum.

Red blood cells

Red blood cells are also called erythrocytes. These are disk-shaped cells produced in the bone marrow. Red blood cells have no nucleus, and their cytoplasm is filled with hemoglobin.

Hemoglobin is a red-pigmented protein that binds loosely to oxygen atoms and carbon dioxide molecules. It is the mechanism of transport of these substances. (Much carbon dioxide is also transported as bicarbonate ions.) Hemoglobin also binds to carbon monoxide. Unfortunately, this binding is irreversible, so it often leads to carbon monoxide poisoning.

A red blood cell circulates for about 120 days and is then destroyed in the spleen, an organ located near the stomach and composed primarily of lymph node tissue. When the red blood cell is destroyed, its iron component is preserved for reuse in the liver. The remainder of the hemoglobin converts to bilirubin. This amber substance is the chief pigment in human bile, which is produced in the liver.

White blood cells

White blood cells are referred to as leukocytes. They are generally larger than red blood cells and have clearly defined nuclei. They are also produced in the bone marrow and have various functions in the body. Certain white blood cells called lymphocytes are essential components of the immune system (discussed later in this chapter). Other cells called neutrophils and monocytes function primarily as phagocytes; that is, they attack and engulf invading microorganisms. About 30 percent of the white blood cells are lymphocytes, about 60 percent are neutrophils, and about 8 percent are monocytes. The remaining white blood cells are eosinophils and basophils. Their functions are uncertain; however, basophils are believed to function in allergic responses.


Platelets are small disk-shaped blood fragments produced in the bone marrow. They lack nuclei and are much smaller than erythrocytes. Also known technically as thrombocytes, they serve as the starting material for blood clotting. The platelets adhere to damaged blood vessel walls, and thromboplastin is liberated from the injured tissue. Thromboplastin, in turn, activates other clotting factors in the blood. Along with calcium ions and other factors, thromboplastin converts the blood protein prothrombin into thrombin. Thrombin then catalyzes the conversion of its blood protein fibrinogen into a protein called fibrin, which forms a patchwork mesh at the injury site. As blood cells are trapped in the mesh, a blood clot forms.

Human Excretory System

The human excretory system functions to remove waste from the human body. This system consists of specialized structures and capillary networks that assist in the excretory process. The human excretory system includes the kidneys and their functional unit, the nephron. The excretory activity of the kidneys is modulated by specialized hormones that regulate the amount of absorption within the nephron.


The human kidneys are the major organs of bodily excretion (see Figure 26-1). They are bean-shaped organs located on either side of the backbone at about the level of the stomach and liver. Blood enters the kidneys through renal arteries and leaves through renal veins.Tubes called ureters carry waste products from the kidneys to the urinary bladder for storage or for release.

The product of the kidneys is urine, a watery solution of waste products, salts, organic compounds, and two important nitrogen compounds: uric acid and urea. Uric acid results from nucleic acid decomposition, and urea results from amino acid breakdown in the liver. Both of these nitrogen products can be poisonous to the body and must be removed in the urine.

Fig.- Excretory System


Details of the human excretory system. Position and allied structures of the kidneys (top). A cross section of the kidney showing the two major portions (left). Details of the nephron, the functional unit of the kidney (right).


The functional and structural unit of the kidney is the nephron. The nephron produces urine and is the primary unit of homeostasis in the body. It is essentially a long tubule with a series of associated blood vessels. The upper end of the tubule is an enlarged cuplike structure called the Bowman's capsule. Below the Bowman's capsule, the tubule coils to form the proximal tubule, and then it follows a hairpin turn called the loop of Henle. After the loop of Henle, the tubule coils once more as the distal tubule. It then enters a collecting duct, which also receives urine from other distal tubules.

Within the Bowman's capsule is a coiled ball of capillaries known as a glomerulus. Blood from the renal artery enters the glomerulus. The force of the blood pressure induces plasma to pass through the walls of the glomerulus, pass through the walls of the Bowman's capsule, and flow into the proximal tubule. Red blood cells and large proteins remain in the blood.

After plasma enters the proximal tubule, it passes through the coils, where usable materials and water are reclaimed. Salts, glucose, amino acids, and other useful compounds flow back through tubular cells into the blood by active transport. Osmosis and the activity of hormones assist the movement. The blood fluid then flows through the loop of Henle into the distal tubule. Once more, salts, water, and other useful materials flow back into the bloodstream. Homeostasis is achieved by this process: A selected amount of hydrogen, ammonium, sodium, chloride, and other ions maintain the delicate salt balance in the body.

The fluid moving from the distal tubules into the collecting duct contains urine. The urine flows through the ureters toward the urinary bladder. When the bladder is full, the urine flows through the urethra to the exterior.


Perspiration, also known as sweating, is the production of fluids secreted by the sweat glands in the skin of mammals.

Two types of sweat glands can be found in humans: eccrine glands and apocrine glands. The eccrine sweat glands are distributed over much of the body.

In humans, sweating is primarily a means of thermoregulation, which is achieved by the water-rich secretion of the eccrine glands. Maximum sweat rates of an adult can be up to 2-4 liters per hour or 10-14 liters per day (10-15 g/min.m2 but is less in children prior to puberty. Evaporation of sweat from the skin surface has a cooling effect due to evaporative cooling. Hence, in hot weather, or when the individual's muscles heat up due to exertion, more sweat is produced. Animals with few sweat glands, such as dogs, accomplish similar temperature regulation results by panting, which evaporates water from the moist lining of the oral cavity and pharynx.