Page 8 - LN-ANATOMY OF FLOWERING PLANT

The main function of sieve element is trans¬location of organic solutes. The callose
(a plant polysaccharide) is present in the perforations in the sieve plates.
It is soluble and disappears when the solute is dilute so that the solute can pass
from one cell to another cell through the pores. Callose reappears and sometimes
closes the pores when solute is less dilute, thus stopping the movement.
(b) Companion Cells These are specialised parenchymatous cells, which are closely
associated with the sieve tube elements. Usually, a single companion cell is found
associated with a sieve tube member.
The cytoplasm of the sieve tube element and companion cells are connected by thin
cytoplasmic strands called plasmodesmata, passing through the pit membranes in
their walls. Companion cells are absent in the phloem of pteridophytes and
gymnosperms. They have albuminous cells.
The companion cells In association with phloem parenchyma play an important role
in the maintenance of a pressure gradient in sieve tubes. They form a link between
sieve tube cells and other cells and regulate the passage of materials.
(c) Phloem Parenchyma The phloem parenchyma is made up of elongated, tapering
cylindrical cells which have dense cytoplasm and nucleus. The cell wall is composed
of cellulose and has pits though the plasmodesmatal connections, which exist
between the cells.
They store food materials and other substances like resins, latex and mucilage. The
phloem parenchyma is absent in most of the monocotyledons.
(d) Phloem Fibres The phloem fibres (bast fibres) are made up of sclerenchymatous
cells. These are generally absent in the primary phloem but are found in secondary
phloem. The cell wall of phloem fibres is quite thick. At maturity, these fibres lose
their protoplasm and become dead. The phloem fibres of jute, flax and hemp have
important economic uses.
The Tissue System
The tissues also vary, depending upon their location in the plant body. Their structure
and function would also be dependent on location. Thus, on the basis of their
structure and location, there are three types of tissue system, i.e., epidermal tissue
system, ground or fundamental tissue system and vascular or conducting tissue
system.
1. Epidermal Tissue System
The epidermal tissue system forms the outermost covering of the whole plant body.
Its various components are epidermal cells, stomata and the epidermal appendages,
i.e., trichomes and emergences.
Epidermis The epidermis (Gr. Epi-upon\ derma -skin) is the outermost layer of the
primary plant body. The epidermal cells vary in shape and size and are compactly
arranged to form a continuous layer. This layer is interrupted by stomata. Sometimes
they are separated by intercellular spaces. It is usually single-layered but is also
multilayered in the aerial roots of orchids and leaves of Nerium and Ficus elastica.