PLANT ANATOMY
class 12 Topics:
concept of tissues
types of tissues:
meristematic and permanent
anatomy of root, stem and leaf of dicot and monocot plants
secondary growth in dicot stem Tissues are the group of cells that carry out certain function in the plant body. For example, for growth in which some cells divide to add the cells that leads to the increase in size of the plant parts, to replace old damaged cells, to provide support, for storage of certain substance, to carry out photosynthesis, for conduction of materials in the plant body along with food, water and minerals etc.
TYPES OF TISSUES: 1. Meristematic or meristem: its role is to increase in the number of cells and help in growth. In the fully developed plant body, the increase in the number of cells takes place at certain points such as tips of stems and roots throughout the whole life of plants. The cells which are added become elongated and enlarged resulting in the enlargement of plant organs. The meristematic tissues are categorised into following types: a. On the basis of origin: primary and secondary. Those which are differentiated during the growth of embryo, are called primary. Those which are developed from the permanent tissues of the mature plants, are called secondary. b. On the basis of function:
Protoderm: gives epidermal tissue system
Procambium: gives vascular tissue system
Ground meristem: gives ground tissue system c.
On the basis of position: Apical meristem: present at root and shoot tip
Intercalary meristem: present at nodes and base of leaves
Lateral meristem: present at lateral position in the stem parallel to stem axis.
2. Permanent tissue: these tissues have fixed function in the plant body. The tissues added by the meristematic tissues develop into permanent tissues. They are of two types: simple- consists of similar cells in a group and complex- consists of dissimilar cells in a group
. EXAMPLES OF SIMPLE PERMANENT TISSUES: Parenchyma:
They mainly form the main bulk of the plant body in the herbaceous plants
. They have thin cellulose wall
The cells are with intercellular spaces that help to collect air
The cells are living and metabolically active.
They have storage function in the plant body such as storage of starch in potato tubers.
The parenchyma that have chloroplasts are called chlorenchyma, the photosynthetic cells, maximum found in the leaves. The parenchyma which have large air spaces are called aerenchyma, the characteristic feature of aquatic plants. They help to collect large amount of air. Collenchyma: Their position is near the epidermal layer in the stems and leaves of dicot plants. They have irregularly deposited cellulose and pectin making these cells strong and carry out the function of support in the plant body, providing tensile strength that is stretchable not easily breakable. Living and metabolically active cells. Sclerenchyma:
They are hard tissue and have supportive function in the plant body
. The substance called lignin deposited on their walls to make the cells strong ( the lignified cells) The fibres are elongated cells like threads and the sclereids are oval and stone like hard so called stone cells. The sclereids have more lignin deposited on their walls than the fibres.
They are dead cells and have empty space or cavity inside which is known as lumen. The lignin is deposited in such a way that it leaves gaps – the pits. The fibres are located in different parts of plant body. The hard parts of plants such as shells of nuts, seed coat, fruit pulp (pear fruit) are occupied by the sclereids.
PERMANENT TISSUES:
Xylem and phloem consist of different types of cells in a group. The xylem is composed vessels, tracheids, parenchyma and fibres. The vessels and tracheids are lignified and dead cell like the fibres. The vessels have wider lumen than tracheids. In the vessels and tracheids, the lignin deposition forms different patterns such as annular (ring like), spiral, scalariform (ladder like) and reticulate (net like). The annular and spiral types have narrow lumen and the other three have wider lumen. The phloem tissue is composed of sieve tubes, companion cells, parenchyma and fibres. The sieve tubes are living with peripherally organised cytoplasm without nucleus. The companion cells provide metabolic support to the sieve tubes as they lack the nucleus