CBSE Class 9 Biology Tissues Notes Set 07

Meristematic Tissue

Meristems are groups of young, living cells that keep dividing. They add new cells to the plant body and so make it grow. The cells are small, thin-walled, closely packed without gaps, and have a large nucleus and dense cytoplasm. They do not store food and have very little vacuole. Meristems are found at the tips of roots and shoots (apical meristems).

Permanent Tissue

When cells formed by meristems stop dividing and take up a specific shape, structure and function, they form permanent tissue. A complex permanent tissue is made of more than one type of cell that work together; xylem is an example.

Xylem

Xylem is the tissue of the vascular bundle that carries water and minerals from the roots upward. It also gives mechanical strength. It is made of four kinds of cells.
1. Tracheids: long, dead cells with tapering ends and thick, lignified walls. They conduct water.
2. Vessels (tracheae): long tube-like dead cells joined end to end. They conduct water.
3. Xylem fibres: dead cells with thick walls that give support.
4. Xylem parenchyma: the only living cells of xylem; they store food.
Tracheids and vessels together are called tracheary elements.

Types of Xylem

Primary xylem forms during the growth of the primary body of the plant. It has protoxylem (formed first) and metaxylem (formed later). Secondary xylem is formed when the plant matures and is abundant in woody trees.
In a dicot stem the protoxylem is towards the centre and the metaxylem towards the outside (endarch arrangement). In a dicot root the protoxylem lies towards the outside and the metaxylem towards the centre (exarch arrangement).

Structure of a Young Dicot Stem

A transverse section of a young dicot stem such as sunflower shows the following parts from outside to inside.
1. Epidermis: the outermost protective layer covered with a cuticle; some cells form hairs.
2. Cortex: lies below the epidermis. The outer part (hypodermis) is made of 2 to 3 layers of collenchyma, the middle part of parenchyma and the innermost layer is the endodermis, whose cells contain starch grains.
3. Stele: the central part inside the endodermis. The pericycle, made of patches of sclerenchyma, gives mechanical support.
4. Vascular bundles are arranged in a ring. Medullary rays are radial rows of cells between the bundles that conduct water and food sideways.
5. Pith (medulla): parenchyma cells with intercellular spaces in the centre of the stem. In sunflower it occupies a large part of the stem.