Page 1 From the last chapter, we recall that all living organisms are made of cells. In unicellular organisms, a single cell performs all basic functions. For example, in Amoeba, a single cell carries out movement, intake of food, gaseous exchange and excretion. But in multi- cellular organisms there are millions of cells. Most of these cells are specialised to carry out specific functions. Each specialised function is taken up by a different group of cells. Since these cells carry out only a particular function, they do it very efficiently. In human beings, muscle cells contract and relax to cause movement, nerve cells carry messages, blood flows to transport oxygen, food, hormones and waste material and so on. In plants, vascular tissues conduct food and water from one part of the plant to other parts. So, multi-cellular organisms show division of labour. Cells specialising in one function are often grouped together in the body. This means that a particular function is carried out by a cluster of cells at a definite place in the body. This cluster of cells, called a tissue, is arranged and designed so as to give the highest possible efficiency of function. Blood, phloem and muscle are all examples of tissues. A group of cells that are similar in structure and/or work together to achieve a particular function forms a tissue. 6.1 Are Plants and Animals Made of Same Types of Tissues? Let us compare their structure and functions. Do plants and animals have the same structure? Do they both perform similar functions? There are noticeable differences between the two. Plants are stationary or fixed – they don’t move. Since they have to be upright, they have a large quantity of supportive tissue. The supportive tissue generally has dead cells. Animals on the other hand move around in search of food, mates and shelter. They consume more energy as compared to plants. Most of the tissues they contain are living. Another difference between animals and plants is in the pattern of growth. The growth in plants is limited to certain regions, while this is not so in animals. There are some tissues in plants that divide throughout their life. These tissues are localised in certain regions. Based on the dividing capacity of the tissues, various plant tissues can be classified as growing or meristematic tissue and permanent tissue. Cell growth in animals is more uniform. So, there is no such demarcation of dividing and non- dividing regions in animals. The structural organisation of organs and organ systems is far more specialised and localised in complex animals than even in very complex plants. This fundamental difference reflects the different modes of life pursued by these two major groups of organisms, particularly in their different feeding methods. Also, they are differently adapted for a sedentary existence on one hand (plants) and active locomotion on the other (animals), contributing to this difference in organ system design. It is with reference to these complex animal and plant bodies that we will now talk about the concept of tissues in some detail. 6 T T T T TISSUES ISSUES ISSUES ISSUES ISSUES Chapter 2020-21 Page 2 From the last chapter, we recall that all living organisms are made of cells. In unicellular organisms, a single cell performs all basic functions. For example, in Amoeba, a single cell carries out movement, intake of food, gaseous exchange and excretion. But in multi- cellular organisms there are millions of cells. Most of these cells are specialised to carry out specific functions. Each specialised function is taken up by a different group of cells. Since these cells carry out only a particular function, they do it very efficiently. In human beings, muscle cells contract and relax to cause movement, nerve cells carry messages, blood flows to transport oxygen, food, hormones and waste material and so on. In plants, vascular tissues conduct food and water from one part of the plant to other parts. So, multi-cellular organisms show division of labour. Cells specialising in one function are often grouped together in the body. This means that a particular function is carried out by a cluster of cells at a definite place in the body. This cluster of cells, called a tissue, is arranged and designed so as to give the highest possible efficiency of function. Blood, phloem and muscle are all examples of tissues. A group of cells that are similar in structure and/or work together to achieve a particular function forms a tissue. 6.1 Are Plants and Animals Made of Same Types of Tissues? Let us compare their structure and functions. Do plants and animals have the same structure? Do they both perform similar functions? There are noticeable differences between the two. Plants are stationary or fixed – they don’t move. Since they have to be upright, they have a large quantity of supportive tissue. The supportive tissue generally has dead cells. Animals on the other hand move around in search of food, mates and shelter. They consume more energy as compared to plants. Most of the tissues they contain are living. Another difference between animals and plants is in the pattern of growth. The growth in plants is limited to certain regions, while this is not so in animals. There are some tissues in plants that divide throughout their life. These tissues are localised in certain regions. Based on the dividing capacity of the tissues, various plant tissues can be classified as growing or meristematic tissue and permanent tissue. Cell growth in animals is more uniform. So, there is no such demarcation of dividing and non- dividing regions in animals. The structural organisation of organs and organ systems is far more specialised and localised in complex animals than even in very complex plants. This fundamental difference reflects the different modes of life pursued by these two major groups of organisms, particularly in their different feeding methods. Also, they are differently adapted for a sedentary existence on one hand (plants) and active locomotion on the other (animals), contributing to this difference in organ system design. It is with reference to these complex animal and plant bodies that we will now talk about the concept of tissues in some detail. 6 T T T T TISSUES ISSUES ISSUES ISSUES ISSUES Chapter 2020-21 Lateral meristem uestions 1. What is a tissue? 2. What is the utility of tissues in multi-cellular organisms? 6.2 Plant Tissues 6.2.1 MERISTEMATIC TISSUE • From the above observations, answer the following questions: 1. Which of the two onions has longer roots? Why? 2. Do the roots continue growing even after we have removed their tips? 3. Why would the tips stop growing in jar 2 after we cut them? The growth of plants occurs only in certain specific regions. This is because the dividing tissue, also known as meristematic tissue, is located only at these points. Depending on the region where they are present, meristematic tissues are classified as apical, lateral and intercalary (Fig. 6.2). New cells produced by meristem are initially like those of meristem itself, but as they grow and mature, their characteristics slowly change and they become differentiated as components of other tissues. Fig. 6.1: Growth of roots in onion bulbs Activity ______________ 6.1 • Take two glass jars and fill them with water. • Now, take two onion bulbs and place one on each jar, as shown in Fig. 6.1. • Observe the growth of roots in both the bulbs for a few days. • Measure the length of roots on day 1, 2 and 3. • On day 4, cut the root tips of the onion bulb in jar 2 by about 1 cm. After this, observe the growth of roots in both the jars and measure their lengths each day for five more days and record the observations in tables, like the table below: Length Day 1 Day 2 Day 3 Day 4 Day 5 Jar 1 Jar 2 Q Apical meristem is present at the growing tips of stems and roots and increases the length of the stem and the root. The girth of the stem or root increases due to lateral meristem (cambium). Intercalary meristem seen in some plants is located near the node. Fig. 6.2: Location of meristematic tissue in plant body Jar 1 Jar 2 TISSUES 69 Apical meristem Intercalary meristem 2020-21 Page 3 From the last chapter, we recall that all living organisms are made of cells. In unicellular organisms, a single cell performs all basic functions. For example, in Amoeba, a single cell carries out movement, intake of food, gaseous exchange and excretion. But in multi- cellular organisms there are millions of cells. Most of these cells are specialised to carry out specific functions. Each specialised function is taken up by a different group of cells. Since these cells carry out only a particular function, they do it very efficiently. In human beings, muscle cells contract and relax to cause movement, nerve cells carry messages, blood flows to transport oxygen, food, hormones and waste material and so on. In plants, vascular tissues conduct food and water from one part of the plant to other parts. So, multi-cellular organisms show division of labour. Cells specialising in one function are often grouped together in the body. This means that a particular function is carried out by a cluster of cells at a definite place in the body. This cluster of cells, called a tissue, is arranged and designed so as to give the highest possible efficiency of function. Blood, phloem and muscle are all examples of tissues. A group of cells that are similar in structure and/or work together to achieve a particular function forms a tissue. 6.1 Are Plants and Animals Made of Same Types of Tissues? Let us compare their structure and functions. Do plants and animals have the same structure? Do they both perform similar functions? There are noticeable differences between the two. Plants are stationary or fixed – they don’t move. Since they have to be upright, they have a large quantity of supportive tissue. The supportive tissue generally has dead cells. Animals on the other hand move around in search of food, mates and shelter. They consume more energy as compared to plants. Most of the tissues they contain are living. Another difference between animals and plants is in the pattern of growth. The growth in plants is limited to certain regions, while this is not so in animals. There are some tissues in plants that divide throughout their life. These tissues are localised in certain regions. Based on the dividing capacity of the tissues, various plant tissues can be classified as growing or meristematic tissue and permanent tissue. Cell growth in animals is more uniform. So, there is no such demarcation of dividing and non- dividing regions in animals. The structural organisation of organs and organ systems is far more specialised and localised in complex animals than even in very complex plants. This fundamental difference reflects the different modes of life pursued by these two major groups of organisms, particularly in their different feeding methods. Also, they are differently adapted for a sedentary existence on one hand (plants) and active locomotion on the other (animals), contributing to this difference in organ system design. It is with reference to these complex animal and plant bodies that we will now talk about the concept of tissues in some detail. 6 T T T T TISSUES ISSUES ISSUES ISSUES ISSUES Chapter 2020-21 Lateral meristem uestions 1. What is a tissue? 2. What is the utility of tissues in multi-cellular organisms? 6.2 Plant Tissues 6.2.1 MERISTEMATIC TISSUE • From the above observations, answer the following questions: 1. Which of the two onions has longer roots? Why? 2. Do the roots continue growing even after we have removed their tips? 3. Why would the tips stop growing in jar 2 after we cut them? The growth of plants occurs only in certain specific regions. This is because the dividing tissue, also known as meristematic tissue, is located only at these points. Depending on the region where they are present, meristematic tissues are classified as apical, lateral and intercalary (Fig. 6.2). New cells produced by meristem are initially like those of meristem itself, but as they grow and mature, their characteristics slowly change and they become differentiated as components of other tissues. Fig. 6.1: Growth of roots in onion bulbs Activity ______________ 6.1 • Take two glass jars and fill them with water. • Now, take two onion bulbs and place one on each jar, as shown in Fig. 6.1. • Observe the growth of roots in both the bulbs for a few days. • Measure the length of roots on day 1, 2 and 3. • On day 4, cut the root tips of the onion bulb in jar 2 by about 1 cm. After this, observe the growth of roots in both the jars and measure their lengths each day for five more days and record the observations in tables, like the table below: Length Day 1 Day 2 Day 3 Day 4 Day 5 Jar 1 Jar 2 Q Apical meristem is present at the growing tips of stems and roots and increases the length of the stem and the root. The girth of the stem or root increases due to lateral meristem (cambium). Intercalary meristem seen in some plants is located near the node. Fig. 6.2: Location of meristematic tissue in plant body Jar 1 Jar 2 TISSUES 69 Apical meristem Intercalary meristem 2020-21 SCIENCE 70 Cuticle Epidermis Collenchyma Parenchyma Phloem Vascular bundle Xylem Fig. 6.3: Section of a stem Cells of meristematic tissue are very active, they have dense cytoplasm, thin cellulose walls and prominent nuclei. They lack vacuoles. Can we think why they would lack vacuoles? (You might want to refer to the functions of vacuoles in the chapter on cells.) 6.2.2 PERMANENT TISSUE What happens to the cells formed by meristematic tissue? They take up a specific role and lose the ability to divide. As a result, they form a permanent tissue. This process of taking up a permanent shape, size, and a function is called differentiation. Differentiation leads to the development of various types of permanent tissues. 3. Can we think of reasons why there would be so many types of cells? • We can also try to cut sections of plant roots. We can even try cutting sections of root and stem of different plants. 6.2.2 (i) SIMPLE PERMANENT TISSUE A few layers of cells beneath the epidermis are generally simple permanent tissue. Parenchyma is the most common simple permanent tissue. It consists of relatively unspecialised cells with thin cell walls. They are living cells. They are usually loosely arranged, thus large spaces between cells (intercellular spaces) are found in this tissue (Fig. 6.4 a). This tissue generally stores food. Activity ______________ 6.2 • Take a plant stem and with the help of your teacher cut into very thin slices or sections. • Now, stain the slices with safranin. Place one neatly cut section on a slide, and put a drop of glycerine. • Cover with a cover-slip and observe under a microscope. Observe the various types of cells and their arrangement. Compare it with Fig. 6.3. • Now, answer the following on the basis of your observation: 1. Are all cells similar in structure? 2. How many types of cells can be seen? In some situations, it contains chlorophyll and performs photosynthesis, and then it is called chlorenchyma. In aquatic plants, large air cavities are present in parenchyma to help them float. Such a parenchyma type is called aerenchyma. The flexibility in plants is due to another permanent tissue, collenchyma. It allows bending of various parts of a plant like tendrils and stems of climbers without breaking. It also provides mechanical support. We can find this tissue in leaf stalks below the epidermis. The cells of this tissue are living, elongated and irregularly thickened at the corners. There is very little intercellular space (Fig. 6.4 b). 2020-21 Page 4 From the last chapter, we recall that all living organisms are made of cells. In unicellular organisms, a single cell performs all basic functions. For example, in Amoeba, a single cell carries out movement, intake of food, gaseous exchange and excretion. But in multi- cellular organisms there are millions of cells. Most of these cells are specialised to carry out specific functions. Each specialised function is taken up by a different group of cells. Since these cells carry out only a particular function, they do it very efficiently. In human beings, muscle cells contract and relax to cause movement, nerve cells carry messages, blood flows to transport oxygen, food, hormones and waste material and so on. In plants, vascular tissues conduct food and water from one part of the plant to other parts. So, multi-cellular organisms show division of labour. Cells specialising in one function are often grouped together in the body. This means that a particular function is carried out by a cluster of cells at a definite place in the body. This cluster of cells, called a tissue, is arranged and designed so as to give the highest possible efficiency of function. Blood, phloem and muscle are all examples of tissues. A group of cells that are similar in structure and/or work together to achieve a particular function forms a tissue. 6.1 Are Plants and Animals Made of Same Types of Tissues? Let us compare their structure and functions. Do plants and animals have the same structure? Do they both perform similar functions? There are noticeable differences between the two. Plants are stationary or fixed – they don’t move. Since they have to be upright, they have a large quantity of supportive tissue. The supportive tissue generally has dead cells. Animals on the other hand move around in search of food, mates and shelter. They consume more energy as compared to plants. Most of the tissues they contain are living. Another difference between animals and plants is in the pattern of growth. The growth in plants is limited to certain regions, while this is not so in animals. There are some tissues in plants that divide throughout their life. These tissues are localised in certain regions. Based on the dividing capacity of the tissues, various plant tissues can be classified as growing or meristematic tissue and permanent tissue. Cell growth in animals is more uniform. So, there is no such demarcation of dividing and non- dividing regions in animals. The structural organisation of organs and organ systems is far more specialised and localised in complex animals than even in very complex plants. This fundamental difference reflects the different modes of life pursued by these two major groups of organisms, particularly in their different feeding methods. Also, they are differently adapted for a sedentary existence on one hand (plants) and active locomotion on the other (animals), contributing to this difference in organ system design. It is with reference to these complex animal and plant bodies that we will now talk about the concept of tissues in some detail. 6 T T T T TISSUES ISSUES ISSUES ISSUES ISSUES Chapter 2020-21 Lateral meristem uestions 1. What is a tissue? 2. What is the utility of tissues in multi-cellular organisms? 6.2 Plant Tissues 6.2.1 MERISTEMATIC TISSUE • From the above observations, answer the following questions: 1. Which of the two onions has longer roots? Why? 2. Do the roots continue growing even after we have removed their tips? 3. Why would the tips stop growing in jar 2 after we cut them? The growth of plants occurs only in certain specific regions. This is because the dividing tissue, also known as meristematic tissue, is located only at these points. Depending on the region where they are present, meristematic tissues are classified as apical, lateral and intercalary (Fig. 6.2). New cells produced by meristem are initially like those of meristem itself, but as they grow and mature, their characteristics slowly change and they become differentiated as components of other tissues. Fig. 6.1: Growth of roots in onion bulbs Activity ______________ 6.1 • Take two glass jars and fill them with water. • Now, take two onion bulbs and place one on each jar, as shown in Fig. 6.1. • Observe the growth of roots in both the bulbs for a few days. • Measure the length of roots on day 1, 2 and 3. • On day 4, cut the root tips of the onion bulb in jar 2 by about 1 cm. After this, observe the growth of roots in both the jars and measure their lengths each day for five more days and record the observations in tables, like the table below: Length Day 1 Day 2 Day 3 Day 4 Day 5 Jar 1 Jar 2 Q Apical meristem is present at the growing tips of stems and roots and increases the length of the stem and the root. The girth of the stem or root increases due to lateral meristem (cambium). Intercalary meristem seen in some plants is located near the node. Fig. 6.2: Location of meristematic tissue in plant body Jar 1 Jar 2 TISSUES 69 Apical meristem Intercalary meristem 2020-21 SCIENCE 70 Cuticle Epidermis Collenchyma Parenchyma Phloem Vascular bundle Xylem Fig. 6.3: Section of a stem Cells of meristematic tissue are very active, they have dense cytoplasm, thin cellulose walls and prominent nuclei. They lack vacuoles. Can we think why they would lack vacuoles? (You might want to refer to the functions of vacuoles in the chapter on cells.) 6.2.2 PERMANENT TISSUE What happens to the cells formed by meristematic tissue? They take up a specific role and lose the ability to divide. As a result, they form a permanent tissue. This process of taking up a permanent shape, size, and a function is called differentiation. Differentiation leads to the development of various types of permanent tissues. 3. Can we think of reasons why there would be so many types of cells? • We can also try to cut sections of plant roots. We can even try cutting sections of root and stem of different plants. 6.2.2 (i) SIMPLE PERMANENT TISSUE A few layers of cells beneath the epidermis are generally simple permanent tissue. Parenchyma is the most common simple permanent tissue. It consists of relatively unspecialised cells with thin cell walls. They are living cells. They are usually loosely arranged, thus large spaces between cells (intercellular spaces) are found in this tissue (Fig. 6.4 a). This tissue generally stores food. Activity ______________ 6.2 • Take a plant stem and with the help of your teacher cut into very thin slices or sections. • Now, stain the slices with safranin. Place one neatly cut section on a slide, and put a drop of glycerine. • Cover with a cover-slip and observe under a microscope. Observe the various types of cells and their arrangement. Compare it with Fig. 6.3. • Now, answer the following on the basis of your observation: 1. Are all cells similar in structure? 2. How many types of cells can be seen? In some situations, it contains chlorophyll and performs photosynthesis, and then it is called chlorenchyma. In aquatic plants, large air cavities are present in parenchyma to help them float. Such a parenchyma type is called aerenchyma. The flexibility in plants is due to another permanent tissue, collenchyma. It allows bending of various parts of a plant like tendrils and stems of climbers without breaking. It also provides mechanical support. We can find this tissue in leaf stalks below the epidermis. The cells of this tissue are living, elongated and irregularly thickened at the corners. There is very little intercellular space (Fig. 6.4 b). 2020-21 TISSUES 71 Fig. 6.4: Various types of simple tissues: (a) Parenchyma (b) Collenchyma (c) Sclerenchyma (i) transverse section, (ii) longitudinal section. Yet another type of permanent tissue is sclerenchyma. It is the tissue which makes the plant hard and stiff. We have seen the husk of a coconut. It is made of sclerenchymatous tissue. The cells of this tissue are dead. They are long and narrow as the walls are thickened due to lignin. Often these walls are so thick that there is no internal space inside the cell (Fig. 6.4 c). This tissue is present in stems, around vascular bundles, in the veins of leaves and in the hard covering of seeds and nuts. It provides strength to the plant parts. Activity ______________ 6.3 • Take a freshly plucked leaf of Rhoeo. • Stretch and break it by applying pressure. • While breaking it, keep it stretched gently so that some peel or skin projects out from the cut. • Remove this peel and put it in a petri dish filled with water. • Add a few drops of safranin. • Wait for a couple of minutes and then transfer it onto a slide. Gently place a cover slip over it. • Observe under microscope. What you observe is the outermost layer of cells, called epidermis. The epidermis is usually made of a single layer of cells. In some plants living in very dry habitats, the epidermis may be thicker since protection against water loss is critical. The entire surface of a plant has an outer covering epidermis. It protects all the parts of the plant. Epidermal cells on the aerial a Intercellular spaces b Wall thickenings Nucleus Vacuole Cell wall Narrow lumen Lignified thick wall c (ii) c (i) Thick lignified walls Fig. 6.5: Guard cells and epidermal cells: (a) lateral view, (b) surface view (a) (b) Guard cell Stoma Epidermal cell Guard cells parts of the plant often secrete a waxy, water- resistant layer on their outer surface. This aids in protection against loss of water, mechanical injury and invasion by parasitic fungi. Since it has a protective role to play, cells of epidermal tissue form a continuous layer without intercellular spaces. Most epidermal cells are relatively flat. Often their outer and side walls are thicker than the inner wall. We can observe small pores here and there in the epidermis of the leaf. These pores are called stomata (Fig. 6.5). Stomata are enclosed by two kidney-shaped cells called guard cells. They are necessary for exchanging gases with the atmosphere. Transpiration (loss of water in the form of water vapour) also takes place through stomata. 2020-21 Page 5 From the last chapter, we recall that all living organisms are made of cells. In unicellular organisms, a single cell performs all basic functions. For example, in Amoeba, a single cell carries out movement, intake of food, gaseous exchange and excretion. But in multi- cellular organisms there are millions of cells. Most of these cells are specialised to carry out specific functions. Each specialised function is taken up by a different group of cells. Since these cells carry out only a particular function, they do it very efficiently. In human beings, muscle cells contract and relax to cause movement, nerve cells carry messages, blood flows to transport oxygen, food, hormones and waste material and so on. In plants, vascular tissues conduct food and water from one part of the plant to other parts. So, multi-cellular organisms show division of labour. Cells specialising in one function are often grouped together in the body. This means that a particular function is carried out by a cluster of cells at a definite place in the body. This cluster of cells, called a tissue, is arranged and designed so as to give the highest possible efficiency of function. Blood, phloem and muscle are all examples of tissues. A group of cells that are similar in structure and/or work together to achieve a particular function forms a tissue. 6.1 Are Plants and Animals Made of Same Types of Tissues? Let us compare their structure and functions. Do plants and animals have the same structure? Do they both perform similar functions? There are noticeable differences between the two. Plants are stationary or fixed – they don’t move. Since they have to be upright, they have a large quantity of supportive tissue. The supportive tissue generally has dead cells. Animals on the other hand move around in search of food, mates and shelter. They consume more energy as compared to plants. Most of the tissues they contain are living. Another difference between animals and plants is in the pattern of growth. The growth in plants is limited to certain regions, while this is not so in animals. There are some tissues in plants that divide throughout their life. These tissues are localised in certain regions. Based on the dividing capacity of the tissues, various plant tissues can be classified as growing or meristematic tissue and permanent tissue. Cell growth in animals is more uniform. So, there is no such demarcation of dividing and non- dividing regions in animals. The structural organisation of organs and organ systems is far more specialised and localised in complex animals than even in very complex plants. This fundamental difference reflects the different modes of life pursued by these two major groups of organisms, particularly in their different feeding methods. Also, they are differently adapted for a sedentary existence on one hand (plants) and active locomotion on the other (animals), contributing to this difference in organ system design. It is with reference to these complex animal and plant bodies that we will now talk about the concept of tissues in some detail. 6 T T T T TISSUES ISSUES ISSUES ISSUES ISSUES Chapter 2020-21 Lateral meristem uestions 1. What is a tissue? 2. What is the utility of tissues in multi-cellular organisms? 6.2 Plant Tissues 6.2.1 MERISTEMATIC TISSUE • From the above observations, answer the following questions: 1. Which of the two onions has longer roots? Why? 2. Do the roots continue growing even after we have removed their tips? 3. Why would the tips stop growing in jar 2 after we cut them? The growth of plants occurs only in certain specific regions. This is because the dividing tissue, also known as meristematic tissue, is located only at these points. Depending on the region where they are present, meristematic tissues are classified as apical, lateral and intercalary (Fig. 6.2). New cells produced by meristem are initially like those of meristem itself, but as they grow and mature, their characteristics slowly change and they become differentiated as components of other tissues. Fig. 6.1: Growth of roots in onion bulbs Activity ______________ 6.1 • Take two glass jars and fill them with water. • Now, take two onion bulbs and place one on each jar, as shown in Fig. 6.1. • Observe the growth of roots in both the bulbs for a few days. • Measure the length of roots on day 1, 2 and 3. • On day 4, cut the root tips of the onion bulb in jar 2 by about 1 cm. After this, observe the growth of roots in both the jars and measure their lengths each day for five more days and record the observations in tables, like the table below: Length Day 1 Day 2 Day 3 Day 4 Day 5 Jar 1 Jar 2 Q Apical meristem is present at the growing tips of stems and roots and increases the length of the stem and the root. The girth of the stem or root increases due to lateral meristem (cambium). Intercalary meristem seen in some plants is located near the node. Fig. 6.2: Location of meristematic tissue in plant body Jar 1 Jar 2 TISSUES 69 Apical meristem Intercalary meristem 2020-21 SCIENCE 70 Cuticle Epidermis Collenchyma Parenchyma Phloem Vascular bundle Xylem Fig. 6.3: Section of a stem Cells of meristematic tissue are very active, they have dense cytoplasm, thin cellulose walls and prominent nuclei. They lack vacuoles. Can we think why they would lack vacuoles? (You might want to refer to the functions of vacuoles in the chapter on cells.) 6.2.2 PERMANENT TISSUE What happens to the cells formed by meristematic tissue? They take up a specific role and lose the ability to divide. As a result, they form a permanent tissue. This process of taking up a permanent shape, size, and a function is called differentiation. Differentiation leads to the development of various types of permanent tissues. 3. Can we think of reasons why there would be so many types of cells? • We can also try to cut sections of plant roots. We can even try cutting sections of root and stem of different plants. 6.2.2 (i) SIMPLE PERMANENT TISSUE A few layers of cells beneath the epidermis are generally simple permanent tissue. Parenchyma is the most common simple permanent tissue. It consists of relatively unspecialised cells with thin cell walls. They are living cells. They are usually loosely arranged, thus large spaces between cells (intercellular spaces) are found in this tissue (Fig. 6.4 a). This tissue generally stores food. Activity ______________ 6.2 • Take a plant stem and with the help of your teacher cut into very thin slices or sections. • Now, stain the slices with safranin. Place one neatly cut section on a slide, and put a drop of glycerine. • Cover with a cover-slip and observe under a microscope. Observe the various types of cells and their arrangement. Compare it with Fig. 6.3. • Now, answer the following on the basis of your observation: 1. Are all cells similar in structure? 2. How many types of cells can be seen? In some situations, it contains chlorophyll and performs photosynthesis, and then it is called chlorenchyma. In aquatic plants, large air cavities are present in parenchyma to help them float. Such a parenchyma type is called aerenchyma. The flexibility in plants is due to another permanent tissue, collenchyma. It allows bending of various parts of a plant like tendrils and stems of climbers without breaking. It also provides mechanical support. We can find this tissue in leaf stalks below the epidermis. The cells of this tissue are living, elongated and irregularly thickened at the corners. There is very little intercellular space (Fig. 6.4 b). 2020-21 TISSUES 71 Fig. 6.4: Various types of simple tissues: (a) Parenchyma (b) Collenchyma (c) Sclerenchyma (i) transverse section, (ii) longitudinal section. Yet another type of permanent tissue is sclerenchyma. It is the tissue which makes the plant hard and stiff. We have seen the husk of a coconut. It is made of sclerenchymatous tissue. The cells of this tissue are dead. They are long and narrow as the walls are thickened due to lignin. Often these walls are so thick that there is no internal space inside the cell (Fig. 6.4 c). This tissue is present in stems, around vascular bundles, in the veins of leaves and in the hard covering of seeds and nuts. It provides strength to the plant parts. Activity ______________ 6.3 • Take a freshly plucked leaf of Rhoeo. • Stretch and break it by applying pressure. • While breaking it, keep it stretched gently so that some peel or skin projects out from the cut. • Remove this peel and put it in a petri dish filled with water. • Add a few drops of safranin. • Wait for a couple of minutes and then transfer it onto a slide. Gently place a cover slip over it. • Observe under microscope. What you observe is the outermost layer of cells, called epidermis. The epidermis is usually made of a single layer of cells. In some plants living in very dry habitats, the epidermis may be thicker since protection against water loss is critical. The entire surface of a plant has an outer covering epidermis. It protects all the parts of the plant. Epidermal cells on the aerial a Intercellular spaces b Wall thickenings Nucleus Vacuole Cell wall Narrow lumen Lignified thick wall c (ii) c (i) Thick lignified walls Fig. 6.5: Guard cells and epidermal cells: (a) lateral view, (b) surface view (a) (b) Guard cell Stoma Epidermal cell Guard cells parts of the plant often secrete a waxy, water- resistant layer on their outer surface. This aids in protection against loss of water, mechanical injury and invasion by parasitic fungi. Since it has a protective role to play, cells of epidermal tissue form a continuous layer without intercellular spaces. Most epidermal cells are relatively flat. Often their outer and side walls are thicker than the inner wall. We can observe small pores here and there in the epidermis of the leaf. These pores are called stomata (Fig. 6.5). Stomata are enclosed by two kidney-shaped cells called guard cells. They are necessary for exchanging gases with the atmosphere. Transpiration (loss of water in the form of water vapour) also takes place through stomata. 2020-21 SCIENCE 72 Recall which gas is required for photosynthesis. Find out the role of transpiration in plants. Epidermal cells of the roots, whose function is water absorption, commonly bear long hair- like parts that greatly increase the total absorptive surface area. In some plants like desert plants, epidermis has a thick waxy coating of cutin (chemical substance with waterproof quality) on its outer surface. Can we think of a reason for this? Is the outer layer of a branch of a tree different from the outer layer of a young stem? As plants grow older, the outer protective tissue undergoes certain changes. A strip of secondary meristem located in the cortex forms layers of cells which constitute the cork. Cells of cork are dead and compactly arranged without intercellular spaces (Fig. 6.6). They also have a substance called suberin in their walls that makes them impervious to gases and water. is a distinctive feature of the complex plants, one that has made possible their survival in the terrestrial environment. In Fig. 6.3 showing a section of stem, can you see different types of cells in the vascular bundle? Xylem consists of tracheids, vessels, xylem parenchyma (Fig. 6.7 a,b,c) and xylem fibres. Tracheids and vessels have thick walls, and many are dead cells when mature. Tracheids and vessels are tubular structures. This allows them to transport water and minerals vertically. The parenchyma stores food. Xylem fibres are mainly supportive in function. Phloem is made up of five types of cells: sieve cells, sieve tubes, companion cells, phloem fibres and the phloem parenchyma [Fig. 6.7 (d)]. Sieve tubes are tubular cells with perforated walls. Phloem transports food from leaves to other parts of the plant. Except phloem fibres, other phloem cells are living cells. 6.2.2 (ii) COMPLEX PERMANENT TISSUE The different types of tissues we have discussed until now are all made of one type of cells, which look like each other. Such tissues are called simple permanent tissue. Yet another type of permanent tissue is complex tissue. Complex tissues are made of more than one type of cells. All these cells coordinate to perform a common function. Xylem and phloem are examples of such complex tissues. They are both conducting tissues and constitute a vascular bundle. Vascular tissue Fig. 6.6: Protective tissue Cork cells Ruptured epidermis Nucleus Cytoplasm Pits Pit (a) Tracheid (b) Vessel (c) Xylem parenchyma Fig. 6.7: Types of complex tissue Sieve plate Sieve tube Phloem parenchyma Companion cell (d) Section of phloem 2020-21Read More
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