The seed is the fertilized mature ovule of an ovary, which contains an embryo, the baby plant, in a dormant state. The seed varies from plant to plant and is characteristic of various plants. A large number of seeds constitute important crude drugs. Examples of such drugs include Strophanthus, Castor, Soya bean, Mustard, Kola, Guarana, Calabar bean, Nux-vomica, Nutmeg, etc.
Macroscopical characters of Seeds
The seed contains the embryo, the future plant, which remains protected externally by a harder covering, called the seed coat. The seed coat usually consists of two layers. The outer colored, hard layer is called the testa, and the inner whitish, thin, membranous layer is known as the tegmen. Certain markings and outgrowths are present on the surface or epidermis of the testa. These markings and outgrowths on the epidermis of the testa of a seed, which constitute its external characters, include:
- The micropyle, a small pore near the tip of the seed.
- The hilum, a scar showing the point of attachment of the seed with its stalk.
- The raphe, a ridge formed by the vascular strand of the stalk on one side of some seeds.
- The aril, a fleshy covering on some seeds, which arise from the hilum and almost completely covers the seed.
- The arillode, a covering similar to aril, but arising from the micropylar edge, as in Cardamon.
- The caruncle, a localized fleshy growth arising from the micropyle, as in Castor seed, and
- The strophiole, a wing-like or barrel-shaped outgrowth along the raphe. Some seeds also possess winged testa and others plume of hairs at the summit of the seed.
The relative position, size and arrangement of these external characters vary from seed to seed. Moreover, the epidermis of the testa in different seeds is made up of different cells:
- A layer of thickened palisade cells constitutes the epidermis of Colocynth and Fenugreek. seeds.
- A layer of sclereids constitutes that of Stramonium, Henbane, Capsicum and Nux-vomica seeds
- A layer containing scattered sclereids either singly or in groups occur in Almond and Peach seeds
- A mucilaginous layer occurs in Linseed and Mustard.
- Characteristic elongated tapering cells occur in the epidermis of Cardamon.
- Thickened lignified cells are present in the epidermis of Lobelia seeds.
- Occasionally stomata may also occur in the epidermis of the seed coat,
e.g. seeds of Gossypium species. Other important external characters of the seeds include long slender trichomes as in Cotton seeds; thick highly lignified stiff trichomes as in Nux-vomica seeds slender threadlike awns with plumes o silky hairs as in Strophanthus. These external charact of the seed coat are o immense diagnostic value in the examination o seeds.
The embryo generally consists of a radicle the rudiment shoot, and one or two cotyledons. The embryo in most seeds is associated with a mass of tissue, called the endosperm albumen, which contains reserve food for the future use by the embryo. The seeds that contain endosperm are known as endospermic or albuminous seeds and the one, which does not contain endosperm, are called non-endospermic or exalbuminous seeds. Seeds of the dicotyledonous plants consist of two cotyledons and those of the monocotyledonous one consist of one cotyledon. Both the dicot and monocot seeds may be albuminous or exalbuminous.
Epidermal characters of Seeds
The various markings and outgrowths on the epidermis of the testa of a seed constitute its external characters. In addition to the various mottling and colouration certain special markings can be observed on the surface of the testa. These markings are the hilum, the raphe, the micropyle, and the chalaza. The relative position, size, and arrangement of which vary from seed to seed. The epidermis of the testa in different seeds is made up of different cells:
- A layer of thickened palisade cells constitutes the epidermis of Colocynth and Fenugreek seeds.
- A layer of sclereids constitutes that of Stramonium, Henbane, Capsicum and Nux-vomica seeds.
- A layer containing scattered sclereids either singly or in groups occurs in Almond and Peach seeds.
- A mucilaginous layer occurs in Linseed and Mustard.
- Characteristic elongated tapering cells occur in the epidermis of Cardamon.
- Thickened lignified cells are present in the dermis of Lobelia seeds.
- Occasionally stomata may also epi d t seeds of Gossypium occur in the epidermis of the seed coat
e.g., seeds of Gossypium species. Other important external characters of the seeds include long slender trichomes as in cotton seeds; thick highly lignified stiff trichomes as in Nux-vomica seeds slender threadlike awns with plumes of silky hairs as in strophanthus. These external characters of the seed coat are of immense diagnostic value in the examination of seeds.
Microscopical characters of Seeds
The structure of the seed coat provides the most characteristic histological features. Some seeds, like those of Strophanthus, Nux vomica, Belladonna, Stramomu, Calabar bean, and Plantains, have only one coat, i.e. the testa only, whereas others, like those of Cardamon, Linseed and Mustard, have two coats, i.e. both testa and tegmen. The testa usually consists of four layers of cells: the epidermis, the pigment layer, the sclerenchymatous layer, and the nutrient layer.
These layers are differently constituted in different seeds. The epidermis is variously developed in different seeds. It may be a layer of palisade cells or a layer of sclereids or a mucilaginous layer or a layer of parenchyma. The epidermis may sometimes possess clothing trichomes and rarely stomata. The pigment layer consists of pigmented cells, which give the colour of the testa. A layer of sclerenchymatous cells is present in the testa of many seeds. Several layers of cells forming a nutrient layer occur in some seeds. The structure and grouping of different layers of the testa often offer diagnostic characters of many seeds.
The endosperm cells are parenchymatous with thin cellulose walls and contain food reserves. In some cases, the cell walls are very inuch thickened due to deposition of hemicellulose on their inner surfaces. The cellular structure of the cotyledons resembles that of the leaves with a layer of palisade cells beneath the epidermis. The reserve food found in the endosperm and the cotyledons constitutes mainly of protein reserves, which occur in the form of aleurone grains. These grains vary in size, shape and structure from seed to seed and thus offer good diagnostic characters of many seeds. The aleurone grains in the seeds frequently occur with globules of fixed oils.
There are a large number of seeds, which constitute pharmaceutically and commercially useful drugs. Some of them are described below in the form of Monographs as representative examples for the students so that they can also describe other seed drugs.