Secretory Structures in Plants | Botany

In this article we will discuss about the secretory structures in plants, explained with the help of suitable diagrams.

The group of cells concerned in secretion of cutin, wax, suberin etc. is generally termed as secretory structures. The term secretion includes excretion and recretion also.

The term secretion implies the act of separation of by-products of metabolism from protoplast. These substances may be stored in insoluble forms within the cell or exuded from it, and have either a special physiological function or no use to the plant and regarded as waste. The removal of waste products of metabolism is defined as excretion.

The process, which eliminates the salt from a cell and thus regulates the ion content of it, is defined as recretion. In the strict sense, the term secretion refers to those end products that take part in the metabolism process (ex. hormones and enzymes). The process of cell wall formation, cutinization, cuticularization, wax deposition, suberization etc. are also the examples of secretion.

Examples of excretion are terpenes, saponins, rubber, tannins, crystals etc. Later findings reveal that the borderline between secretion and excretion is ill defined.

As for example, the terpenes are regarded as excretions, though they play important roles in:

(i) Attracting pollinators,

(ii) Repelling animal foes,

(iii) Blocking wounds in plant organs and

(iv) Having antiseptic properties etc.

Therefore, the present day authors use the term secretion to denote excretion and recretion as well. Many of the end products have enormous commercial value, e.g. rubber, opium, gutta-percha etc.

The secretory structures vary greatly in structure and position. They may be either simple glandular trichomes or multicellular glands with vascular tissues. They may be external when originate from epidermis or deep seated or internal such as laticifers and resin ducts. The various structures, both external and internal, involved in secretion are discussed below.

External Secretory Structure:

These include glandular trichomes, nectary, osmophores, hydathodes and salt glands, of which hydathodes and salt glands.

Glandular Trichomes:

These trichomes consist of a stalk with a head above. The stalk may be unicellular or multicellular and in the latter case the cells may be arranged in several rows. The head is the secretory part and may be composed of single cell (ex. Pelargonium) or many cells (e.g. Callitriche). The head is covered with a cuticle. The secretion is accumulated beneath the cuticle.

This type is exampled by the volatile oils like camphors, balsams, peppermint oil, resins etc. Trichomes may also secrete nectar (ex. stipules of Vicia sepium) and water (ex. leaves of Hygrophila). Several of the secretions of the glandular trichomes have role in plant defense. Some are repellent to insects. The glandular hairs of tomato and wild potato (Solatium berthaultii) provide resistance to aphids.

These trichomes secrete sticky exudates that trap the aphids. Plant breeders are now trying to raise a hybrid of cultivated potato that possesses these useful aphid-trapping glandular hairs.

The glandular trichomes of insectivorous plants have specialized functions:

(i) They secrete mucilage that is effective in trapping insects;

(ii) They contain proteolytic enzymes;

(iii) They have absorptive functions, i.e. the products of digestion move into the leaf through them.

Nectary:

Nectary can be defined as a gland or part of a flower that secretes nectar to the exterior of plants. They are divided into floral and extrafloral nectaries. The former is situated within the flower and is directly involved in pollination; the latter occurs on the vegetative organs and is not directly associated with pollination. The nectaries are present on the epidermis.

They may be deeply sunken, or raised above over an outgrowth, or at the level of the organ that bears them. The tissues that compose the nectary are commonly known as nectariferous tissues. These tissues may not form any anatomically differentiated structure – termed non-structural nectaries (e.g. on leaves of Dracaena reflexa, in bracts of Sansevieria zeylanica, on tepals of Cattleya percivaliana etc.).

In some cases it is difficult to distinguish the non-structural nectaries from the surrounding tissues. The histochemical test reveals that the parenchyma cells composing the non­structural nectaries have a high acid phosphatase activity like the anatomically differentiated ones. The nectariferous tissues may form anatomically differentiated structure termed structural nectaries, which are macroscopically recognizable also.

It consists of stalk cells and secretory cells with cuticle. The cells of the nectary are specialized parenchyma cells that are small, thin walled and contain dense cytoplasm, dictyosomes, endoplasmic reticulum, small vacuoles and large nuclei (Fig. 11.1). The nectaries may be provided with special vascular tissues. The nectar is supplied by phloem and it accumulates between the cuticle and secretory cells.

The nectar of non-structural nectaries is exuded through stomata. Nectar, in structural nectaries, is secreted by epidermal cells or trichomes directly to the outside. The nectar contains glucose, sucrose and fructose as major components. The followings are also recorded in the nectar of different plant species: maltose, melobiose, mucilage, proteins, phosphates, mineral ions, organic acids, oxidases, sucrose, vitamins etc.

The non-enzymatic proteins are detected in the floral nectary of Erica, Bergenia etc. The lipids are reported from Jacaranda, Trichocereus etc. The nectar also contains all the essential aminoacids, which with their smell attract some of the anthophilous insects. It is suggested that nectar provides the aminoacid requirements for the insects. The cells of nectaries may absorb the sugary fluid. So they possess both secretory and absorptive properties.

The extra floral nectaries are very common in dicotyledonous and much less in monocotyledonous plants. It rarely occurs in Poaceae (e.g. Andropogon and Eragrostis). In dicots it is present on all organs, namely —in the cotyledons (e.g. Ricinns communis), on leaf margins (e.g. Rosa, Populus etc.), on phyllodes (e.g. Acacia longifolia), on pulvinus (e.g. Thunbergia grandiflora), on stipules (ex. Vicia fava) etc.

Osmophores:

Osmophores can be defined as certain special areas on floral organs, which differ in structure from the neighbouring cells and have the fragrance producing properties to attract pollinators.

The fragrance of a flower is due to volatile low molecular weight terpenes. ‘The fragrant materials are not exuded by the glandular trichome. This substance occurs as droplets in the cytoplasm of epidermal cells. The oil droplets of terpenes, at an appropriate temperature diffuse out of the cell in gaseous form through the cell wall and cuticles.

As a result the fragrance is produced. In many plants certain localized cells are only involved in producing the fragrant materials. The cells differ markedly from the other normal cells situated nearby and are termed as osmophores. 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; they can be stained with neutral red and thus can be identified from the neighbouring cells. Osmophores are present in Asclepiadaceae, Araceae, Orchidaceae, Aristolochiaceae and Liliaceae.

The osmophores of Ceropegia (Fig. 11.2) 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.

Internal Secretory Structure:

The internal secretory structures may be composed of a single cell or groups of cells. They may occur throughout a tissue (e.g. oils or enzymes) or may be localized in distribution.

Example: castor oil obtained from the endosperm of Ricinus; ground nut oil extracted from the cotyledons of Arachis; the source of palm oil is the mesocarp of the fruit of Elaeis guineensis; the seed of Carthamus 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 officinale, the oil cells secreting the aromatic oil occur in the phloem of Cinnamomum zeylanicum.

Glands and Ducts:

They comprise a group of cells or sometimes a single cell that is readily distinguishable from the neighbouring cells and secretes a specific substance. These cells are thin walled with dense protoplasm and sometimes occur as layer surrounding a cavity, known as secretory cavity.

The secretion is discharged and accumulates within the internal cavity. These cavities may be more or less spherical or much elongated like tubes and respectively termed as glands or ducts.

The internal cavities originate by three ways and accordingly the following three types of glands are recognized:

(i) Schizogenous glands:

These are formed by the dissolution of middle lamella, thus separating apart the cells to form cavity. Example: oil glands of Eucalyptus, the secretory ducts of Rhus glabra, resin duct of Pinus etc. This cavity remains surrounded by a ring of intact parenchyma cells, termed epithelium, which forms a well-defined boundary of the gland.

(ii) Lysigenous glands:

These glands originate by lysis of a few cells thus forming the cavity (ex. glands present in the leaves and fruits of Citrus sp., that are also formed schizogenously).

(iii) Schizolysigenous glands:

These glands arise by both phenomenon schizogeny and lysis, i.e. the middle lamella and some of the adjoining cells disintegrate to form cavity. Example: Eugenia caryophyllata where the glands are present in the floral parts that are the source of oil of clove. In contrast to schizogenous gland the lysigenous — and schizolysigenous gland have no clear-cut boundary.

Laticifers (Fig. 11.3):

Laticifers can be defined as a specialized cell or a row of such cells that secrete the milky fluid termed latex. The word laticifer is used as a general term to denote the various latex-secreting structures — latex cell, latex vessel, latex duct, latex tube and laticiferous duct. The laticiferous duct is a cavity into which latex is secreted.

The latex cell may be simple or branched and is derived from the enlargement of a single cell. The latex ducts are elongated, branched and aseptate. The latex vessel is simple or branched tube that usually anastomoses with similar tubes; it is formed as a result of enlargement and union of chain of cells. The latex tube usually means either a latex cell or the latex vessel.

The borderline between the different terms of latex secreting structures (i.e. latex cell, -duct, -vessel, -tube etc.) is ill defined and so the word laticifer is introduced as a general term. The laticifers may be simple or compound on the basis of origin. The simple laticifer is derived from a single cell whereas the compound laticifer originates from a longitudinal file of cells.

Latex is fluid produced in the latex vessels or cells. It is usually white and milky (ex. Euphorbia, Asclepias, Lactuca etc.), yellow and brown (e.g. Cannabis), orange and sometimes colourless and clear (e.g. Morus, Nerium etc.). It contains a number of substances including sugars, proteins, alkaloids, oils, mineral salts, organic acids, terpenes, resins, rubber etc.

The latex of Euphorbia milii contains starch grains that are dumb-bell shaped. The proteolytic enzyme papain is present in the latex of Carica papaya. The latex of Asclepias syriaca contains the enzyme pectinase. The latex of some Euphorbia species is rich in vitamin B₁.

The cell wall of laticifers is thick and may be thicker than the adjacent cells. They are not lignified. The growth of cell wall occurs through apposition process. The tip of latex cell is thin walled. The walls are composed of cellulose, hemicellulose and pectin.

The laticifers may occur throughout the plant body or their distribution is restricted to certain tissues. Laticifers are grouped into two: non-articulate and articulate. The former is derived from the enlargement of a single cell. This cell has the potentiality of unlimited and rapid growth, and elongates to form long latex tubes. The tubes may remain unbranched termed non-articulate unbranched laticifer (e.g. Vinca, Cannabis, Urtica etc.).

In some plants (e.g. Euphorbia, Nerium etc.) the tubes may branch called non-articulate branched laticifers that seldom anastomose with similar types. The non-articulate laticifers are coenocytic and multinucleated, and also termed as laticiferous cell.

There is continuity of laticifers between the shoots and branches. Laticifers grow through the intercellular spaces and the enzyme pectinase helps in the process. Pectinase is secreted by the growing tip of laticifers and dissolves the middle lamella.

The articulate laticifers, also termed laticiferous vessel, consist of longitudinal files of cells. The transverse end walls of the individual cell either remain intact or break down partly or wholly to form a continuous tube —the latex vessel. So the articulated laticifers are always compound in origin. They occur in primary or secondary phloem and may be present in cortical parenchyma.

They may remain either as a single chain of cells without any anastomosis termed articulated non-anastomosing laticifer (e.g. Convolvulus, Allium, Musaetc.), or they may anastomose with similar file of cells to form a complex anastomose system called articulated anastomosing laticifer (e.g. Lactuca, Papaver, Caricn papaya etc.).

The enzyme cellulase is found in the latex of articulated laticifers and it is absent from non- articulated laticifers. So it is suggested that cellulase is involved in the lysis of common transverse walls during the formation of articulated laticifers.

Latex occurs in 900 genera distributed in 20 families of mostly in dicotyledons ( e.g. Apocynaceae, Asclepiadaceae, Compositae, Euphorbiaceae, Papaveraceae etc.) and in a few families of monocotyledons (e.g. Araceae, Musaceae and Liliaceae).

They are very much economically important, a few of which are mentioned below: 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 gutta yields gutta-percha. The latex of Achras sapota yields chicle, from which chewing gum is made.

Function of laticifers:

Regarding the function of laticifers different views were expressed since 1877, a brief account of which is mentioned below:

(1) It is a vital sap vessel and similar to the blood vessels of animals.

(2) It takes part in translocation of assimilates as it is associated with phloem.

(3) It stores food materials.

(4) It is now considered as secretory tissues where the secretory substances do not re-enter the plant metabolism.

(5) Sen and Chawan (1972) suggested that laticifiers regulate the water balance in plants.

(6) It has role in the transport of oxygen.

(7) It has role in healing up of wounds.

(8) It acts as a defense against herbivores and microorganisms. [The human ocular tissues are damaged by the latex of Calotropis procera].

(9) McCay and Mahlberg (1973) reported the absence of bacterial activity from laticifers of Asclepias in vivo.

Myrosin Cell:

It is a special type of secretory cell where the enzyme is produced and stored. Myrosin cells are idioblasts, which contain the enzyme myrosinase (β-thioglucosidase). They are reported from Cruciferae, Capparidaceae, Moringaceae, Tropaeolaceae etc.

They are more or less similar in shape to the neighbouring cells and may be elongated or assume various shapes. They occur in the integument of seeds, in the exocarp and endocarp of fruits, in the cotyledons, in the cortex of stem, in the leaf lamina, spongy parenchyma etc.

The enzyme myrosinase hydrolyses thioglucosides to produce mustard oil and other substances. The enzyme and substrate occur in different cells, i.e. the enzyme is produced in myrosin cells and thioglucosides are present in the other parenchyma cells.

The reaction between the enzyme and substrate is brought about only when the tissues are damaged and thus mustard oil is released. When insects feed on such plants mustard oil is released in the insect gut. It is demonstrated that mustard oil is toxic to a number of insects.