NCERT Class 9 Science Exploration Chapter 03 Tissues in Action PDF Download

Chapter 3: Tissues in Action

Life begins when a single cell divides itself several times to give rise to a large number of cells. These cells gradually form the skin (protection), muscles (movement), bones (support), nerves (control and coordination), and all other organs. This process is so intricate that it is considered one of nature's greatest engineering marvels. Researchers have been trying to understand, replicate and modify this process for human welfare. To do so, it is essential to understand the natural biological processes that govern growth and development in plants and animals. What makes cells group together to form tissues? Why do some tissues grow throughout life while others do not?

In Chapter 2, Cell: The Building Block of Life, you have learnt that the cell is the basic unit of life. Many cells come together to form a multicellular organism. In all multicellular organisms, there is a hierarchy of organisation. Cells of similar type performing similar function group together to form a tissue, more than one type of tissues form organs, different organs form organ systems and organ systems form an organism. In unicellular organisms, such as amoeba, a single cell performs all functions of life. In multicellular organisms like plants and animals, different groups of cells perform different functions. A tissue is a group of cells (similar in structure) that work together to perform a specific function. The formation of different types of tissues leads to the division of labour, which increases the efficiency of the body and enables it to carry out complex life processes. For example, in animals, muscle tissue enables movement and nervous tissue carries messages to different parts of the body. In plants, conducting tissues, such as xylem transports water and minerals, while phloem transports food.

Think It Over

• How is the study of cells and tissues significant for understanding the life processes and human welfare?
• How are tissues in plants and animals different, and why?
• How is the division of labour at various levels of organisation in multicellular organisms correlated with their structure and function?

3.1 Why are Plant and Animal Tissues Different?

In Chapter 2, Cell: The Building Block of Life, you compared plant and animal cells, and studied the major differences between them. Most plants are fixed in one place and do not move from place to place like animals. They need support to stay firm and upright. Plant cells have a cell wall that provides rigidity and strength. In general, animals can move (although some, such as sponges, are immobile). Without a rigid cell wall, animal cells can change shape easily. This cellular flexibility eventually helps make their bodies suitable for locomotion.

Another major difference between plants and animals is their mode of nutrition. Animals have tissues that help them digest food obtained from different food sources, while plants have tissues that help them utilise solar energy for synthesising the food components through photosynthesis. Plants and animals have distinct tissues for transporting food and water to different parts of the body. The growth patterns in plants and animals also vary because the tissues responsible for growth differ in structure and function. In this chapter, we will learn how the structures of plant and animal tissues relate to their specific functions.

Teacher's Note

The key reason plant and animal tissues are different is their lifestyle: plants stay in one place and need rigid support, while animals move around and need flexibility. When you see a plant cell or animal cell in a diagram, ask yourself what job that structure does. The cell wall makes plants stiff and strong; the lack of it lets animal cells bend and squeeze. This difference drives everything about their tissues.

3.2 Tissues for Growth in Plants

You must have observed that a small seedling grows into a tall tree, roots grow deep into the soil, stems become thicker with time and grass grows again after being eaten by grazing animals. Which tissues are responsible for these changes?

Plants grow in different ways -

• increase in length (height of stem and depth of roots),
• increase in girth (thickness of stem), and
• regrowth after cutting the branches or grazing by animals.

This growth require actively dividing cells that together form a tissue called a meristematic tissue.

Let us explore different kinds of meristematic tissues.

3.2.1 Apical meristem - How do plants grow in length?

Let us study the growth of roots in an onion bulb.

Activity 3.1: Let us design experiments

1. Take two glass jars or couplin jars and fill them with water.
2. Now, take two onion bulbs and place one in each jar, as shown in Fig. 3.1.
3. Observe the growth of roots in both bulbs for a few days.
4. Measure the length of roots on days 1, 2 and 3.
5. On day 3, cut the root tips of the onion bulb in Jar B by about 1 cm. After this, observe the growth of roots in both the jars, measure their lengths for four more days (day 4 onwards), and record your observations in Table 3.1.

[Figure 3.1: Experimental set-up to observe the growth of roots, See in your textbook]

What trend do you observe in the data you recorded in Table 3.1? Are your observations similar to those presented in the graphical representation (Fig. 3.2)? What do you infer?

• Roots in Jar A continue to grow in length.
• Roots in Jar B stop growing after the tips are cut.

This shows that roots grow only from their tips. The tips consist of cells which divide continuously.

You may recall observing mitosis in onion root tips in Chapter 2, Cell: The Building Block of Life. This observation confirms that root tips contain actively dividing cells. Similarly, shoot tips also contain actively dividing cells that help the shoots to grow in length. Thus, we conclude that plants have growth zones at the tips of their roots and shoots, called the apical meristems, which help the plants grow in length (Fig. 3.3).

[Figure 3.2: Growth of onion roots, See in your textbook]

[Figure 3.3: Location of apical meristem in a sapling, See in your textbook]

Teacher's Note

In Activity 3.1, when you cut off the root tip in Jar B, growth stops because you have removed the apical meristem - the only place where that root can grow longer. This tells you that growth in length happens only at the tips, not along the whole root. Remember this when you see questions about where plants grow: it is always at the apical meristems at the tips of roots and shoots.

3.2.2 Lateral meristem - How do plants grow in girth?

Look at your surroundings. You must have observed that the stems of dicot plants not only grow in length but also increase in diameter or girth over time. What causes this increase in girth? One possible explanation may be the activity of meristematic tissues. If you have visited a timber yard or observed the cut trunk of a tree, you may have noticed several ring-like patterns on the cut surface of the wood (Fig. 3.4). These are annual growth rings. Some annual rings are wide and some are narrow, reflecting the favourable or unfavourable growth conditions during a particular year. By counting these annual rings, scientists can estimate the age of a tree and also understand the climatic conditions under which it grew.

[Figure 3.4: T.S. of a tree trunk showing annual growth rings, See in your textbook]

Key Points

• A tissue is a group of similar cells that work together to perform a specific function, allowing organisms to carry out complex life processes through division of labour.
• Plant and animal tissues differ because plants are fixed in place and need rigid cell walls for support, while animals move and need flexible cells without cell walls.
• Apical meristems at the tips of roots and shoots contain actively dividing cells that enable plants to grow in length.
• Lateral meristems in dicot plants cause growth in thickness or girth, and create annual growth rings that record the age and growing conditions of a tree.

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