PHLOEM | Complex Permanent Tissue | Anatomy of Flowering Plants | Part 6 | NEET | AIIMS | CSIR-NET | NCERT BIOLOGY Class 11 .
Phloem (Bast) : Term “Phloem” was given by Nageli. Its main function is the transport of organic food materials from leaves to stem and roots. It consists of the following elements –
(i) Sieve tubes
• They are long tube-like cells placed end to end, forming a continuous channel.
• Their cell wall is made up of cellulose.
• Their transverse wall is perforated like a normal sieve and hence they are called as sieve tubes.
• They are anucleated living cells.
• Callose pad may be visible in the winter season. P-proteins are also found.
(ii) Companion cells
• They are thin-walled cells which are associated with sieve tubes by plasmodesmata.
• They contain nucleus and are therefore living in nature.
• They are not found in Pteridophytes and Gymnosperms but are always present in angiosperms.
(iii) Phloem fibres
• Phloem fibres (bast fibres) are made up of sclerenchymatous cells. These are generally absent in the primary phloem but are found in the secondary phloem. Their main function is mechanical support.
(iv) Phloem parenchyma : Their function is storage of food material. They are absent in monocots.
(3) Special tissues or secretory tissue : These tissue perform special function in plants, e.g. secretion; of resins, gum, oil and latex. These tissues are of following types –
(a) Laticiferous tissue : Some plants contain a milky juice, known as latex. Latex is a highly complex substance, comprising proteins, enzymes, carbohydrates, oils, rubber, poisons, mostly alkaloids and even some inorganic salts. Laticiferous tissue are of two types.
(i) Laticiferous cells : A laticiferous cells is a very highly branched cell with long slender processes ramifying in all directions in the ground tissue. Laticiferous vessels are formed by the fusion of rows of cells, mainly along the length of the organ.
(b) Glandular tissue : This is a highly specialized tissue consisting of glands.
(i) External glands : These are situated on the surface of the organs. Usually, they consist of diverse types of hairs. e.g. Urtica dioica.
(ii) Internal glands : These perform diverse types of functions, including secretion of essential oils, mucilage, gums, resin, etc.
(iii) Lysigenous cavities : These are formed by disintegration of groups of cells in a tissue. e.g., the oil glands in the ring of orange.
(iv) Schizogenous cavities : These are formed by splitting of cell walls and separation of masses of cells example, the resin canals of conifers.
(v) Hydathodes or Water stomata : These occur at the tips of veins in the leaves of many water plants and some mesophytic land plants. The structure of the water stomata is similar to the stoma of a land plant but the guard cells of the former do not have the capacity to close. A water stoma is a permanently open pore. They are responsible for guttation.
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