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
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
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:
Ring like - Annular
Network like - Reticulate
Twisted - Spiral
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
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
They differ from each other with respect to certain plant structures, such
Large, pinnately compound
Male cones are large, compact
while female cones are loose with
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.