In this essay we will discuss about the external and internal secretory structures of plant tissues.
Essay # 1. External Secretory Structures of Plant Tissues:
External surfaces of the plant bear several secretory structures of epidermal origin or epidermal derivatives or emergences from deeper tissues. These include glands, glandular trichomes, nectaries, osmophores, hydathodes and salt glands.
a. Glandular Trichomes:
The epidermal surfaces of leaves and flowers of many plants contain glandular trichomes and glands. The glandular trichomes consist of a unicellular or multicellular stalk with a head above. The cells of multicellular stalk may be arranged in several rows.
The unicellular (e.g. Pelargonium) or multicellular (e.g. Callitriche) heads of the glandular trichomes perform the actual function of secretion. The secretion may remain accumulated beneath the cuticle cover of the head such as the volatile oils like camphors, balsams, peppermint oil, resins etc. Glandular trichomes may also secrete nectar and water.
Glandular trichomes also play a role in the defence mechanism of the plant. Certain secretions may act as insect repellents. The glandular hairs of tomato and wild potato provide resistance to aphids by secreting sticky exudates that trap the aphids.
The glandular trichomes of insectivorous plants secrete mucilage to stick the insects for digestion. They also secrete proteolytic enzymes and absorb the protein digestion products into the leaf.
b. Nectary:
Nectary is defined as a gland or any floral part that secretes nectar to attract the pollinators. Sometimes, it may occur on the vegetative organs and is not directly concerned with pollination. These structures are epidermal outgrowths or deeply sunken or at the level of the epidermis of the organ.
The nectariferous tissues may not form any anatomically differentiated structure known as non-structural nectaries (e.g., Dracaena reflexa leaves, bracts of Sansevieria zeylanica, tepals of Cattleya percivaliana etc.). Histochemically, the cells of the non-structural nectaries can be distinguished. These cells show high acid phosphatase activity like the structural ones.
Anatomically and morphologically, differentiated nectariferous tissues are called structural nectaries. These structures have stalks and specialised parenchymatous secretory cells with cuticle.
The nectary cells are small, thin-walled with dense protoplasts containing dictyosomes, endoplasmic reticulum, small vacuoles and large nuclei. The structures have vascular supplies through which nectar is supplied and accumulated between the cuticle and secretory cells.
Exudation of nectar from non-structural nectaries takes place through stomata whereas in structural nectaries it takes place trough epidermal cells or trichomes directly to the outside.
The nectar contains glucose, sucrose and fructose as major components. In addition maltose, melobiose, mucilage, proteins, phosphates, mineral ions, organic acids, oxidases, sucrose, vitamins, essential amino acids, etc. are also found to be present in the nectar. The essential amino acids attract the insects for pollination.
The extra floral nectaries are very common in dicots and much less in monocots. It rarely occurs in Poaceae (e.g., Andropogon and Eragrostis). In dicots it is present on all the organs.
c. Osmophores:
Osmophores are some special areas on floral organs, which differ in structure from the neighbouring cells and produce fragrance to attract insects. The fragrance of a flower is due to volatile low terpenes which are not exuded by the glandular trichome. These substances at an appropriate temperature diffuse out of the cell in gaseous form through the cell wall and cuticles to produce fragrance.
The fragrance producing cells differ markedly from the adjacent normal cells. The osmophores secrete terpenes as the main fragrant materials. In some species belonging to Araceae the fragrant substance may contain amines and ammonia in addition to terpenes.
The osmophores appear as flaps, cilia or brushes and can be stained with neutral red. Osmophores are present in Asclepiadaceae Araceae, Orchidaceae, Aristolochiaceae and Burmaniaceae.
The osmophores of Ceropegia consists of an epidermal layer and two rows of isodiametric cells situated below the epidermis. The epidermis contains dense cytoplasm and the isodiametric cells are filled with starch grains. The starch disappears after the emission of fragrant materials.
Essay # 2. Internal Secretory Structures of Plant Tissues:
The internal secretory structures may be composed of a single cell or of groups of cells. They may occur throughout a tissue (e.g., oils or enzymes) or may be localised in distribution.
Example:
Castor oil obtained from the endosperm of Ricinus; groundnut oil extracted from the cotyledons of Arachis; the source of palm oil in the mesocarp of the fruit of Elaeis guiensis; the seed of Carthemus tinctorius yields safflower oil etc., Sometimes resin or oil-secreting idioblasts are formed, e.g., oleo-resin cells are present in the ground tissue of the rhizome of Zingiber officinales, the oil cells secreting the aromatic oil occur in the phloem of Cinnamomum zeylanicum.
a. Glands and Ducts:
Glands and ducts are formed by a group of cells or even by a solitary cell readily distinguishable from the neighbouring cells. These structures secrete or accumulate a specific substance. Cells of glands and ducts are thin-walled with dense protoplasm and sometimes occur as layer surrounding a cavity, known as secretory cavity.
The cavity may be spherical or elongated to a tube-like structure and termed as gland or duct, respectively. The secretion product is discharged and accumulated within this cavity.
The cavity may originate:
(i) Schizogenously,
(ii) Lysigenously and
(iii) Schizolysigenously.
Schizogenous glands are formed by the dissolution of middle lamella separating apart the cells to form cavity (e.g., oil glands of Eucalyptus, the secretory ducts of Rhus glabra, resin duct of Pinus etc.)
The schizogenous cavity is lined by a layer of intact parenchyma cells, termed epithelium. Lysigenous glands originate by lysis of a few cells forming the cavity (e.g., glands on leaves and fruits of Citrus). Schizolysigenous glands arise through schizogeny and lysis (e.g., Eugenia caryophyllata the source of clove oil). These schizolysigenous glands are devoid of definite boundaries.
b. Laticifers:
Laticifers are the specialised cell or row of cells that secrete the milky or watery fluid termed latex. The term encompasses the various structures like latex cell, latex vessel, latex duct, latex tube and laticiferous duct. The laticiferous duct is a tubular cavity into which latex is secreted and remains stored.
A single vegetative cell may be converted into a simple or branched latex cell. The latex ducts are also modifications of the vegetative cells into aseptated, elongated and branched structures. The latex vessel is usually an anastomosing tubular structure. It may also be simple or unbranched.
The latex vessel is formed as a result of enlargement and fusion of a group of cells. Based on origin the laticifers may be simple or compound. The simple laticifer is derived from a single cell whereas the compound laticifer originates from a longitudinal pile of cells.
The cell wall of the laticifer is non-lignified but thicker than the adjacent cells. Of course, the latex cell tip is thin-walled. The cell walls of laticifers grow in apposition and are composed of cellulose, hemicellulose and pectin.
Latex is produced within the latex vessels or cells. It is usually white and milky (e.g., Euphorbia, Asclepias, Lactuca etc.), yellow and brown (e.g., Cannabis), orange and sometimes colourless and clear (e.g., Morus, Nerium etc.). It contains many substances like sugars, proteins, alkaloids, oils, mineral salts, organic acids, terpenes, resins, rubber etc.
The latex of Euphorbia milii contains dumb-bell shaped starch grains. The latex of Carica papaya contains the proteolytic enzyme papain. The latex of Asclepias syriaca contains the enzyme pectinase. The latex of some Euphorbia species is rich in vitamin B1.
The laticifers, where present, may remain distributed throughout the plant body or may be confined to certain tissues. Laticifers may be non-articulated and articulated. The former, which is derived from the enlargement of a single cell, has the potentiality of unlimited and rapid growth, and elongates to form a long unbranched latex tube (e.g., Vinca, Cannabis, Urtica etc.).
In some plants (e.g., Euphorbia, Nerium etc.) the non-articulated latex tubes may be branched. The non-articulated laticifers are coenocytic and multinucleate, and also termed as laticiferous cell. There is continuity of laticifers between the shoots and branches. The laticifers grow through the intercellular spaces with the help of the enzyme pectinase secreted by the growing tips of the laticifers.
The articulate laticifers or the laticiferous vessels, consist of longitudinal pile of cells. The transverse end walls of the individual cells may remain intact or partly or totally obliterated to form a continuous tube called the latex vessel. Therefore, they are of compound origin. They occur in primary or secondary phloem and in cortex.
The articulated laticifers may remain as a single chain of cells without anastomosis — articulated non-anastomosing laticifer (e.g., Convolvulus, Allium, Musa etc.). They may also form a complex anastomosing system called articulated anastomosing laticifers (e.g., Lactuca, Papaver, Carica papaya etc.). The enzyme cellulase is found in the latex of articulated laticifers suggesting that it may be involved in the lysis of common transverse walls during development.
Latex occurs in 900 genera distributed in 20 families, mostly in dicotyledons (e.g., Apocyna- ceae, Asclepiadaceae, Compositae, Euphorbiaceae, Papavaraceae etc.) and in a few families of monocotyledons (e.g., Araceae, Musaceae and Liliaceae). The different types of latex are of great economic value.
The opium, a medicinally important alkaloid, is obtained from Papaver somniferum. The most important latex is rubber whose principal source is Hevea brasiliensis. The species of Palaquium yields gutta-percha. The latex of Achras sapota yields chicle, from which chewing gum is made.
c. Hydathodes:
Hydathodes are water stomata, water pores or water glands on leaf tips and margins. Water exudation through hydathodes takes place in slowly transpiring well-watered plants. Cells of the hydathode are small, thin- walled with dense cytoplasm and are devoid of chloroplasts.
The intercellular space system within the tissue is very extensive. Such tissue is called epithem. There is xylem supply to the epithem and water moves through the intercellular spaces of the epithem to the outside through the pores. This process of water exudation is called guttation.
In some cases the cells of the hydathode secrete water and these hydathodesare sometimes considered as glands. Water stomata are modified stomata that are incapable of closing.
Therefore, hydathodes are of two types:
(i) Epidermal hydathode and
(ii) Epithem hydathode.
The epidermal hydathodes secrete ions and minerals along with water. They are now referred to as salt glands. Epithem hydathodes, on the other hand, are generally referred to as hydathodes or water stomata.
Hydathode occurs in many families like Poaceae, Araceae, Ranunculaceae, Papavaraceae etc. including pteridophytes (Equisetum). They may occur over the entire surface of the leaf or may be restricted to margins or tips and even at the tips of tendrils (e.g., Vitis vimfera).
A typical hydathode consists of:
(a) Water pore,
(b) Epithem, and
(c) Tracheids.
(a) Water Pore:
Each hydathode opens to the exterior through one or more pores termed water pores which are generally regarded as modified stomata with guard cells. Each guard cell is chloroplast-free with a large nucleus, many mitochondria and a large vacuole.
The guard cells are generally devoid of closing mechanics and, therefore, the pores always remain open with the exception in Impatiens and Tropaeolum where the water pores show diurnal periodicity of opening and closing.
The water pore internally opens to a chamber termed sub-stomatal chamber lined by one or two layers of large isodiametric cells easily distinguishable from neighbouring epithem cells (Fig. 5.66).
(b) Epithem:
Below the sub-stomatal chamber this parenchymatous tissue remains Individual cells of this tissue are thin-walled, lack chloroplasts and some cells contain some small vacuoles. The epithem cells of Taraxacum officinale Papaver rhoeas develop wall protuberances which are called transfer cells. Extensive intercellular spaces occur in the epithem tissue except Crassula argentata.
(c) Tracheid:
The tracheids of the xylem supply terminate at the intercellular spaces of the epithem and they even reach the sub-stomatal chamber (e.g., Primula vulgaris) or even they may extend up to the water pore (e.g., Apomogeton distachyus). Usually 1 to 3 veins fuse with each other before reaching the epithem. The terminal tracheids usually have spiral, scalariform or annular thickenings.
Function:
The main function of hydathode is water exudation in the liquid form. A Colocasia leaf may exude 10 to 100 ml of water per night. Root pressure is the main cause of guttation. The exuded fluid may be pure water or it may contain ammonium phosphate, magnesium chloride, calcium nitrate etc. in low quantities. The occurrence of amino acids, sugars and water-soluble vitamins in the fluid are reported in the leaves of Zea mays and Colocasia antiquarum.