NCERT Class 9 Science Tissues

Tissues

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 and respiratory gases, respiration and excretion. But in multi-cellular organisms there are millions of cells. Most of these cells are specialised to carry out a few 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. Most of the tissues they have are supportive, which provides them with structural strength. Most of these tissues are dead, since dead cells can provide mechanical strength as easily as live ones, and need less maintenance.

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.

6.2 Plant Tissues

6.2.1 MERISTEMATIC TISSUE

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:

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

Please refer to attached file for NCERT Class 9 Science Tissues