Secretory Structures in Plants PDF

SECRETORY STRUCTURES Secretory structures are concerned with secretion of essential oils, tannins, terpenes, mucilage, latex, resins, water, salt, enzymes and hormones. Usually, the secreted substances are metabolic by- products or substances of special physiologic functions as enzymes and hormones....

SECRETORY STRUCTURES Secretory structures are concerned with secretion of essential oils, tannins, terpenes, mucilage, latex, resins, water, salt, enzymes and hormones. Usually, the secreted substances are metabolic by- products or substances of special physiologic functions as enzymes and hormones. Secretory structures may be internal or external. INTERNAL SECRETORY STRUCTURES Are found inside the plant body. 1-Secretory Cells (secretory idioblasts.) - The simplest secretory structure is a single cell where it is distinguished from adjacent non- oil cell of Gossybium secretory cells. - Secretory cells contain various substances. Some plants, (e.g.Gossybium‫ ) القطن‬have oil secretory idioplasts which appear as enlarged parenchyma cells. - Mucilaginous idioplasts are common in Malvaceae. - Tanniferous idioplasts contain tannins (phenol derivatives). Tannins are used in tanning (‫)الدباغة‬, dyeing and preparation of ink. Tannins are considered as metabolic byproducts. - Tannins can be found in fruits like blackberries and cranberries, tea, nuts, grapes, apples and legumes beans, dark chocolate and cocoa. blackberry ) ‫) التوت األسود‬ - Astringent (‫(الزع أو قابض‬taste, which we feel when eating unripe fruit, is the effect of tannins. They attach to the proteins in the saliva. Transverse section of tannin cell from pine needle (Pinus strobus), showing a huge tannin deposit in the cell vacuole. Punica granatum 02 2-Secretory cavities and ducts. - Secretory cavities and ducts consist of large, intercellular spaces that are formed either by the disintegration of cells (lysigenous cavity) or by the separation of the walls of neighboring cells (schizogenus cavity). - Cavities occur as spherical spaces while ducts are more elongated space. They can also called resin, oil or gum ducts. A-Lysigenous ducts - Are surrounded by disintegrated cells the breakdown of which leads to the formation of these ducts. - As an example for these cavities, is the Citrus oil cavities. B-Schizogenous ducts - These spaces are lined with secretory cells, as an epithelium that produces resin. - In high resin-yielding plants, multilayered epithelium is formed. - Usually, the epithelium is surrounded with a sheath of unlignified thick- walled parenchyma. - Conifers contain resin ducts in vascular and ground tissues of all plant organs. In some conifers, the resin ducts develop only as a result of injury. Gummosis (formation of gum) is formed by transformation of some organized substances like starch and cell wall components into an amorphous state. kino veins In the wood of Eucalyptous , there are special type of ducts contain polyphenols known as kino veins. Presence of polyphenols leads to formation of the Red Bloodwood. 02  NATURE OF RESIN The word resin comes from French resin and from Latin resina. Resin is a hydrocarbon secretion of many plants, (particularly coniferous trees). It is amorphous products of complex mixtures of essential volatile oils, volatile terpenes with lesser components (carboxylic acids and others) of dissolved non-volatile solids which make resin thick and sticky. - Resins seal over wounds, protecting the plant from pests and infections. - Resins have been used for food glazing, waterproofing, varnishes, adhesives, perfume, incense, medicines, and many other purposes. - Pine oil is used for fragrance, flavoring, and in cleaning agents to give the “pine” odor. SOME EXAMPLES OF RESIN A-Gum resin - Is a natural gum used as a fragrance, perfume, medicinal and incense. B-Hard resin - Mastic (Mistica) is an example of hard resin; obtained from the Mediterranean Mastic Tree (Pistacia lentiscus). - Mastic was commonly used in ice creams, puddings, pastries, nougat, sauces, soups, fruit and vegetable preserves, soft drinks, coffee, and many other food and medicinal uses. 00 3-Laticifers - Laticifers are a secretory systems secrete latex. The latex may be olorless, white, yellow, yellow-orange, yellow-brown or red. Natural rubber latex - Latex is a suspension or emultion, the chemical constitution of which differs in different plants. Among the suspended materials in latex; there are rubber particles (C5H5n), waxes, alkaloids proteins, The rubber tree Hevea brasiliensis essential oils, mucilage, tannins, organic acids a member of Euphorbiaceae and occasionally starch grains also found. - Natural rubber constitutes about 50% of total rubber production while the other 50% is synthetic petroleum derivatives. - Rubber is used in many significant industrial and medicinal uses. Papaver somniferum - latex exuded from the capsule fruits of Papaver somniferum (Opium poppy) contains the medically important alkaloids morphene, codaeine and papaverene. latex 02 Types of laticifers Laticifers are divided into main types: a-Non-articulate ( laticiferous cells ) -Develop from single cells which greatly elongate with the growth of the plant. -It may be branched or unbranched. Branched unbranched b-Articulate (laticiferous vessels) -Develop from series of cells which also elongated. It may also branched or unbranched. -The end walls of such cells remain Entire or become porous or disappear completely. Branched unbranched EXTERNAL SECRETORY STRUCTURES Are found on the surface of the plant body. 1-Hydathodes - The liquid drops that are exuded from the leaves are always through special structures called water stomata (hydathodes). - Structurally, hydathodes consist of a simple pores in the epidermal layer found at the leaf apex and margin. - A space found just behind the pore is surrounded by a parenchymatous tissue (epithelium). 02  - The xylem elements of the leaf veins lets terminate in this tissue; so the water with other soluble components is forced out of the xylem elements. - As a result, the space behind the water stomata (hydathode) gets filled with the water which is pushed out of the pore. - This process by which plants loose water in liquid form is called guttation (e.g. Tomato, Grasses, Colochasia, Cucurbita members, etc.) while the water is lost from the plant body in the form of vapors by transpiration. - Guttation takes place only under high relative humidity in the atmosphere and soil. 2- Nectaries "nectar-secreting trichomes". - Nectarines may be structural or non- structural. Structural nectaries consists of specialiezed tissues which differ in structure from the neighboring epidermal tissue. - Structural nectaries may be in the form of non-glandular unicellular epidermal hairs or in the form of glandular structures. - The nectariferous tissue usually contains branches of vascular bundles with high portion of phloem elements. - Nectaries may develop on all parts of the plant. It can be distinguished into floral which develop on the floral parts and 02  extrafloral which develop on the vegetative parts like petioles, stipules and leaf margins. - Nectar solution (contains sucrose, glucose, fructose, organic acids, proteins, mucilage,) is secreted by vesicles derived from the endoplasmic reticulum which release their contents by the fusion with the plasmalemma. - The nectariferous tissue is generally composed of small cells with thin walls, relatively large nuclei, dense granular cytoplasm and small vacuoles. Exudation of nectar tacks place by diffusion through the cell wall by rupture the cuticle. 3-Glandular epidermal trichomes - Glandular trichomes are an important source of essential oils (natural fragrances) and products can be used by the pharmaceutical industries. - Glandular trichomes are multicellular secretory structures; either sessile or stalked.The stalked glandular trichomes composed of a non-secretory stalk and unicellular or multicellular secretory head. Sessile secretory gland Of Stalked glandular trichomes Stalked glandular trichomes Lavandula angustifolia of Salvea sclarea of Tomato 02  Cuticle - The secreted substances are commonly released between the wall of the secretory cell and its cuticle which eventually burst. - In some cases, the secreted substances are released through pores in the cuticle. A-salt secreting trichomes - Salt glands occur in a number of halophytic species, e.g. Avicennia marian, (mangrove). - Each gland consists of 2 - 4 collecting cells (C), a stalk cell (St) and 8 – 12 secretory cells that discharge the salt through pores in the cuticle to allow excretion of a concentrated salt solution from mesophyll tissues to leaf surfaces to regulate their internal salt load. - Salt glands of halophytic monocotyledons are structurally simpler than those of dicotyledons Avicennia marian leaves and usually consist of just two cells. - After salt secretion, the glands die and the salt is deposited on the leaf surface as a white powdery layer. B-Mucilage- secreting trichomes - Such epidermal glandular trichomes secrete stiky mucilage for capturing insects and often small animals e.g. the carnivorous plant Drosera. In addition, such glands secrete digestive proteiolytic enzymes which act up on the insect body. Then the digestion products can be absorbed by the plant tissues. 02 C-Collators - Collators are multicellular secretory structures that may occur on vegetative or reproductive shoot apices. - Their probable role is to protect meristems by secreting a viscous sticky secretion consists mainly of terpenes and mucilage. - The developing meristems (e.g.scaly buds) are completely covered with colleter exudate which protects it. A scaly bud D-Stinging trichomes - Stinging trichomes of Urtica have bladder-like glandular base and needle-like upper part. - The base is inclosed by elevated epidermal cells. - The wall of the needle-like part is impregnated with silica while the base wall is impregnated with calcium. - On contact and penetrating the skin, the needle tip breaks off injecting the irritating fluid of the glandular bas. The irritating substances in that fluid are histamine and acetyle coline. 02 E-Terpen-secreting trichomes - These glandular secrete essential volatile oils which are irritating to predatory insects. - Trichomes of tomato (Solanum lycopersicum) are particularly abundant on the leaves and stems and accumulate monoterpenes and sesquiterpenes, mostly in the form of insecticidal carboxylic acid derivatives. - In mint (Mentha), the glandular trichomes secrete essential oils consisting of largely p-menthanes monoterpenes that include menthone and menthol. - Tobacco glandular trichomes produce diterpenoids. D-Osmophors - Many popular flower scents come from the entire surface of the petals, where scent glands called osmophores are found. - The flower scents are complex of volatile essential oils. Osmophores on a style. - Volatile essential oils obtained from flowers are not only secreted by glandular hairs (osmophores) but also diffuse through the cytoplasm, the cell Osmophores on petals walls and the cuticle to the atmosphere. 02