Secretion refers to the complex phenomena of separation of substances from the protoplast or their isolation in parts of the protoplast. The secreted substances may be surplus ions, which are removed in the form of salts, surplus assimilates eliminated as sugars, compounds that may or may not be end products of metabolism but are not utilizable or partially utilizable physiologically (alkaloids, tannins, terpenes, resins) subtances that have a special physiological function after they are secreted (enzymes, hormones).
The tissues which are concerned with the secretion of gums, resins, volatile oils, nectar, latex and other substances are called secretory tissues. These are further subdivided into two groups – I. Laticiferous tissue and II. Glandular tissue.
I. Laticiferous tissue:
This tissues is concerned with the secretion of a thick fluid called latex. Latex is an emulsion of sugars, proteins, gums, alkaloids, enzymes, rubber, etc,. It is a viscous fluid and is colloidal in nature. The latex may be milky, yellowish or colourless and stored in vacuoles.
The latex of some plants is of great importance, especially as a source of rubber ( Haevea brassiliensis – para rubber and Ficus elastic – Indian rubber), Chicle (Achras sapota), Papain ( Carica), Morphine, Codeine (Papaver somniferum).
Latex secreting cells are known as laticifers. They consist of living cell with several nuclei. The cell walls are non-lignified and show elasticity. Laticifers are generally distributed throughout the plant body but more commonly associated with phloem.
The laticifers are of two types – Latex cells or non-articulate latex ducts and latex vessels or articulate latex ducts.
Latex cells or Non-articulate latex ducts:
These are isolated laticifers which extend as branched structures for long distances in the plant body. They originate as minute structures, elongate and ramify in all directions of the plant body by repeated branching, but they do not fuse together, thus not netted structures are formed.
The walls of the ducts are soft and thick. Free nuclear division occurs in them, resulting in coenocytic condition. They may be branched or unbranched. Branched non-articulate laticifers commonly occur in leaves. They pass through the vascular bundles, ramify in mesophyll, and often reach the epidermis.
They are commonly found in Calotropis, Euphorbia, Nerium, Vinca, Cannabis.
Latex Vessels or Articulate latex ducts:
They are formed by a series of cells whose end walls dissolve to form elongated structures. They originate in the meristems from rows of cells. These series of cells become united by dissolution of intervening walls. They grow more or less as parallel ducts which by means of branching and frequent anastomoses form a complex network. They resemble a xylem vessel but they are living and coenocytic.
They are found in Papaveraceae, Compositeae, Euphorbiaceae, Moraceae. They are frequently associated with phloem. In Carica papaya the laticifers apparently occur not only in the phloem but also in the xylem. In Hevea brasiliensis which yields economically important rubber these are developed in the secondary phloem.
II Glandular Tissues:
This tissue consists of special structures, the glands. These glands contain some secretory are excretory products. The glands may consist of isolated cells or group of cells with or without central cavity. The glands which lie embedded in the interior tissues of plant body are called as internal glandular tissues, e.g., oil glands, mucilage secreting, gum secreting, resin, tannin secreting, digestive glands. The glands which occur on the epidermis are called as external glands, e.g., nectaries.
Oil glands:
These are internal glands which frequently contain essential oils in them. These oils are volatile and odoriferous. They are formed due to breaking down of cells containing volatile oils. On the disintegration of the cells the oil gets stored in the large cavities of glands. These cavities are lysogenious in nature. Ex – oil glands of Citrus, Eucalyptus
Glands secreting resins, gums, etc.
In the gymnosperms and in many angiospermic families, resins, gums,oils and many other substances are secreted and conducted in ducts. In many gymnosperms, e.g., Pinus, these ducts or canals form extensive system extending both vertically and horizontally. In Umbeliiferae the ducts are local in occurrence and limited in extend.
These glands are schizogenous in nature, and when mature, have the structure of tube with an epithelial lining. These glands are internal.
Osmophors:
In some plants, the fragrance of flowers is due to special glands called osmophors. Ex- Orchids
Digestive Glands:
Insectivourous plants growing in nitrogen deficient soils depend on organisms like insects for their nitrogen requirements. In order to attract the insects and digest the insect proteins, the leaves of these plants modify into trap or insectivorous leaves. Digestive glands are formed in these leaves which secrete protein-digesting enzymes. With the help of these enzymes the insect proteins are digested and the nitrogenous compounds are assimilated into the plant.
In Nepenthes (pitcher plant) the lamina of the leaf is modified into a pitcher. The upper part of the pitcher, on the inner side, has large number of digestive glands. The insect is killed and digested by the action of proteolytic enzymes produced by digestive glands.
In Drosera (sundew), the digestive glands are at the tips of the leaf tentacles.
Nectaries or Nectar Glands:
Many insect pollinated plants produce nectar which attracts insects. This nectar is secreted by special cellular structures, the nectaries.
The nectary tissue many be restricted to the epidermis, or it may e several layers of cells deep. The nectaries are covered on the outside with a cuticle. Vascular tissue occurs more or less close to the secretory tissue. A close relation exists between the relative amount of phloem in the vascular tissue supplying the nectary and the concentration of sugar in the nectar. If phloem predominates, the nectar may have upto 50 percent of sugar; if xylem predominates, the content of sugar may fall as low as 8 percent.
The cells in nectaries have dense cytoplasm and small vacuoles. Nectaries are usually formed in the flowers which are called floral nectaries. The floral nectaries occupy various positions on the flower; they are found on sepals, petals, stamens, ovaries or the receptacle. Ex- Hibiscus, Cucurbita, Euphorbia, Citrus.
When nectaries are present on the plant parts, other than flowers, they are called extrafloral nectaries. The extrafloral nectaries occur on stem, leaves, stipules and pedicels of flowers. Ex – in Passiflora they are found on the petiole.
Hydathodes:
In some plants, water exudate in form of dorplets from the leaf margins or apices. This phenomenon is called guttation. Guttation occurs due to the action of secretory tissues called hydathodes or water stomata.
Structurally, hydathodes are modified parts of leaves, usually located at the leaf margins or tips, in which water released from the xylem is enabled to reach the surface of the leaf. The hydathode consists of vein ending, epither cells, cavity and pore.
The vein ending consists of tracheids associated with loose mass of cells called epithem. This is followed by a cavity which opens outside through pores in the epidermis. These pores are called water stomata that have guard cells without opening and closing mechanism.
From the vein ending the water moves into the cavity through the epithem cells and is finally forced out through the pores. This phenomenon occurs particularly on a humid cool morning that follows a warm night under conditions of low transpiration and abundant soil moisture.
Guttation can be noticed in plants like Colacasia, Lycopersicon, Pothos. The water droplets shine in the slanting sun rays and are generally referred to as dew drops.
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