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MHT-CET : Biology Entrance Exam

MHT - CET : Biology - Conquest of Land Know More

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Vascular Tissues: Xylem and phloem are the conducting tissues which are also called vascular tissues. Xylem carries out conduction of water and minerals while phloem is concerned with conduction of food and other organic substances.

Xylem: It is also called 'wood'. It is a permanent complex tissue. It consists of more than one type of cell. Xylem is composed of four elements, namely, tracheids, vessels, xylem fibres (sclerenchyma) and xylem parenchyma.

Tracheid: It is an elongated cell which is without nucleus and cytoplasm. A large cavity inside it is called lumen. Tracheids have tapering ends and tracheids show deposition of lignin. These thickenings are in different forms such as:

1.      Ring like - Annular

2.      Network like - Reticulate

3.      Twisted - Spiral

4.      Ladder like - Scalariform

Sometimes the lignin deposition is not uniform. It is absent in a number of circular areas. These are called 'pits'. If these pits are with well developed borders, they are called bordered pits. In the course of evolution, xylem first appeared as tracheids.

Vessels: They are tube like cylindrical structures, formed by cells arranged in a row and placed end to end. There are no cross walls between the cells. Thus, a continuous pipe like structure is formed which along with conduction gives mechanical support to the plant. Vessels are found in angiosperms.

Phloem: Phloem consists of sieve tubes, sieve cells, companion cells and phloem parenchyma. The sieve elements are living cells but at maturity they lack nucleus.

Sieve Cells: They are long, thin-walled and slender. They have cellulose cell walls. The sieve cells are arranged end to end to form cylindrical sieve tubes. Sieve plates are the special sieve areas which are developed between two cells. These sieve areas help in the conduction of carbohydrates. Attached to the sieve tube are small cells with nucleus and cytoplasm. These are called companion cells. They control the activities of sieve tubes.

Albuminous Cells: They are the cells present in gymnosperms. These cells are attached to the sieve cells and they perform the functions of companion cell.

Gymnosperms are divided into two classes, namely:

a) Cycadophyta and
b) Coniferophyta

They differ from each other with respect to certain plant structures, such as:

Character

Cycadophyta

Coniferophyta

Stem

Unbranched

Well-branched

Leaves

Large, pinnately compound

Simple leaves

Male and
Female cones

Male cones are large, compact
while female cones are loose with
large ovules.
Example: cycas.

Male as well as female
cones are compact.
Example: Sequoia, Pine.


Evolution of Xylem: The xylem first appeared as tracheids. In angiosperms and gymnosperms, the tracheids show pits. Only tracheids are found in pteridophytes and gymnosperms. In angiosperms, xylem vessels are the main xylem elements.

Evolution of Phloem: In pteridophytes and gymnosperms, the phloem exists as sieve cells. In angiosperms, it is in the form of sieve tube elements in association with companion cells.

In jowar, new roots develop near the base of the radicle as the primary root survives only for a short time. These new roots are called 'seminal roots'. These roots constitute the fibrous root system.

Xerophytic Characters in Cycas: Cycas leaflet has a thick cuticle to reduce transpiration. Stomata are present only on the lower epidermis and in the pits. The leaflets show only midrib, the lateral veins are absent. Transfusion tissues are present.

In cycas, the embryo is surrounded by nutritive tissue called endosperm. Here, the endosperm is haploid female gametophyte. Cycas seed is naked and shows two cotyledons. After germination of cycas seed, either a male or female cycas plant is produced.

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