The glandular tissue comprises glandular outgrowths on different parts such as leaves, stem, flowers etc.
These may be epidermal or sub-epidermal in origin.
These are of two types as given below:
1. External Glands and
2. Internal Glands
1. External Glands:
These glands are present on the epidermis, usually in the form of outgrowths.
These are of several types as given below:
(a) Hydathodes. (Water stomata):
Hydathodes are special organs found in some herbaceous angiosperms such as Tomato, Colocassia, Cicer and some grasses. Through these structures water exudes in the form of drops. The exudation of water is also called guttation. Generally, they are present at the tips or margins of leaves that grow in humid places. Water droplets are found in morning hours, fog high absorption & low transpiration due to root pressure.
Hydathode is a complex structure. It is situated at the ends of veins. It is always open. At the ends of tracheids, large numbers of loosely arranged cells are present. The cells are living, have interred cellular spaces and are filled with water. This tissue is called as epithem. Above the epithem there is an air cavity, which opens out through a pore present in epidermis. The pore is guarded by guard cells.
Water stomata are morphologically comparable to stomata but they differ from stomata in following features:
1. These are situated at the end of veins
2. The pores always remain open.
3 These are larger in size.
4. The guard cells ape smaller in size and devoid of chloroplasts.
5. Guard cells do not have closing and opening mechanism.
6. These exude impure water in liquid form.
(b) Glandular Hairs:
The unicellular or multicellular glandular hairs are found distributed on different parts of the body. They have a unicellular or multicellular stalk and a head possessing one or more secretory cells. They secrete various substances, such as poisons, mucilage and oils. Such hairs are found in plants like tobacco (Nicotiana), Plumbago, Boerhaavia etc.
(c) Stinging Hairs:
In some plants, such as Urticadioica, albuminoidal poison secreting unicellular hairs are present on the lower surface of leaves. They cause irritation, if we touch the plant, by injecting poison into the skin. These hairs protect the plant from grazing animals.
(d) Nectaries/ Nectar Secreting Glands:
These occur mainly in the flowers of insect pollinated members. The epidermal layer of a gland is composed of columnar cells with denser protoplasm. They secrete their products. The products accumulate on the surface of the gland in the form of droplets. As the glands are present in between floral parts, they are called as floral nectaries. The extra floral nectaries are also reported on vegetative parts such as on leaves of Cassia and Ricinus, Extrafloral nectaries are present on rim of the pitcher of Nepenthes, on the stipules of Catheranthus etc.
(e) Digestive Glands:
The insectivorousplants such as Drosera. Pinguicula, Nepenthes, Utriculuria, Dionaea and Aldrovanda possess certain digestive glands. In Nepenthes, the digestive glands are present at the base of the pitcher. Generally the glands secrete different enzymes such as proteolytic enzymes, acid phosphatases, esterases etc., to digest captured insects for nitrogen supply.
2. Internal Glands:
There are many plants that possess glands embedded in various tissues of roots, stems, leaves, flowers and fruits.
These are of the following types:
(a) Lysogenous Cavities:
These cavities are formed by the disintegration of group of secretory cells. Disintegrating secretory cells have big-vacuole and protoplasm. The vacuole stores secretory products. After disintegration the products are released into the cavity. The cavity acts as reservoir. The lysogenous cavities in the rind of Citrus fruits and leaves of Eucalyptus store essential oils.
The fragrance of flowers is produced by some volatile essential oils. The essential oils are mainly present in the epidermal cells of perianth lobes. But in some families, such as Asclepidiaceae and Orchidaceae, the fragrance originates in some special glands called osmophores. Various floral parts may be modified as osmophores. Prolonged spadix portion of Araceae, tongue shaped petal of ‘Platanthera and Dendrobium are some of the examples of this kind.
(c) Mucilages Secreting Glands:
These glands are found in the leaves and secrete mucilage, e.g., Piper betle, Cruciferae members etc.
(d) Shizogenous Cavities or Ducts:
These cavities are formed by the enlargement of intercellular spaces in the tissues. Resin canals of conifers secretory ducts of Compositae and Umbelliferae are examples for such type of cavities. The cavity is surrounded by epithelial layers. This layer is composed of glandular cells which contain denser cytoplasm and prominent nucleus. The epithelial cells secrete their products such as resins, tannins etc., into the cavities. These canals may be connected with each other to form inter communicating system.