The eye is a complex sensory organ that allows us to see the world around us. The eye works by gathering light from the environment and converting it into electrical signals that can be interpreted by the brain. The basic structure of the eye includes the cornea, iris, pupil, lens, retina, and optic nerve. The cornea is the transparent outer layer of the eye that helps to focus incoming light. The iris is the colored part of the eye that regulates the amount of light that enters the eye through the pupil, which is the black circular opening in the center of the iris. The lens is a clear, flexible structure located behind the iris that helps to focus light onto the retina. The retina is a thin layer of tissue that lines the back of the eye and contains photoreceptor cells called rods and cones. These cells convert light into electrical signals that are sent to the brain via the optic nerve. The rods are responsible for the low light vision and detecting motion, while the cones are ...
Term Pteridophyta was proposed by Hackel. The study of pteridophytes is known as pteridology. Pteridophytes are known as reptiles of the plant kingdom. They are also called as vascular cryptogams. They are vascular plants, i.e. xylem and phloem are present in it. In pteridophytes, vessels in xylem and companion cells in phloem are absent. Pteridophytes are used for medicinal purposes and as soil binders. They are also frequently grown as ornamentals. Pteridophytes are more adapted terrestrial plants as compared to bryophytes. Because vascular tissue is present in them and they have roots. Pteridophytes are not completely successful terrestrial plants because they need water for fertilization, so pteridophytes grow in cool, shady and moist places. In pteridophytes, the plant body is completely differentiated into root, stem and leaves. The primary root of pteridophytes remains alive for a short period. After some time it is replaced by adventitious roots. The stem is erect or prostrate. when in pteridophytes stem is underground which is known as the rhizome. On the basis of leaves, pteridophytes are of two types. First in which stem is smaller while leaves are larger. They are known as megaphyllous pteridophytes e.g. Ferns. Second, in which stem is larger and leaves are smaller. They are called microphyllous pteridophytes e.g. Selaginella.
The life cycle of pteridophytes :
The plant is sporophyte i.e. diploid and they reproduce by spore formation. Most of the pteridophytes are homosporous i.e. only one type of spores are formed during reproduction. But some pteridophytes are heterosporous i.e. two types of spores microspores and megaspores. The formation of spore takes place in sporangia. Sporangia are formed at the abaxial surface of leaves. The leaves on which sporangia are formed are called sporophylls (reproductive leaves) and normal photosynthetic leaves are called tropophylls (vegetative leaves) Sporangia are present in groups, these groups are called sorus (plural-sori) Sori are found on sporophylls. In Pteridophyta, sporophylls are also photosynthetic. This is a unique character of Pteridophyta. Spore mother cells are present in sporangia. Spores are formed within the sporangium by meiosis in spore mother cells and these spores start the gametophytic generation. In Pteridophyta, the germination of spores is exosporic i.e. germination takes place outside the sporangia (in soil). In soil, an inconspicuous, small and multicellular gametophyte is formed by the germination of each spore, which is known as prothallus. The formation of gametophyte takes place in the soil therefore it is free (independent) and mostly photosynthetic. These gametophytes require cool, damp, shady places to grow. Because of this specific restricted requirement and the need for water for fertilization, the spread of living pteridophytes is limited and restricted to narrow geographical regions.
(1) Psilopsida :
The most ancient vascular plants are placed in this class. The plants in this class have many primitive characters. Their plant body is differentiated into the stem (rhizome), scaly leaves and rhizoids. Only one living genus is present in this class - Psilotum = A living fossil. e.g. Rhynia and Horneophyton = Fossil plants.
(2) Lycopsida :
Club mosses are placed in this class. The plant body of club mosses is differentiated into root, stem and leaves. Leaves are green and photosynthetic and known as tropophylls. Sporophylls are present in group at the tip of the plant. This group is called strobilus and cone. E.g. Lycopodium and Selaginella.
(3) Sphenopsida :
In this class, Horsetails are included. The plant body of horsetails is differentiated into root, stem or rhizome and scaly leaves. Silica is present in the epidermis of these plants. Due to this, the surface of the plant becomes rough. If two horsetail plants collide, then there is a dangerous chance of fire in the forest. The formation of sporangia takes place on special branches of stem called sporangiophores. Sporangiophores are arranged in a group and form a long and condensed cone. This cone is formed at the apical part of the aerial stem. E.g. Equisetum.
(4) Pteropsida :
This is the largest group of pteridophytes. They are commonly called as ferns. Most of the pteridophytes are ferns. Ferns are megaphyllous i.e. rhizome is small and leaves are comparatively larger and these leaves are known as 'Fronds'. Multicellular hair is present on the young leaf and young rhizome of ferns which are called ramenta. They are for protection. There is no difference between tropophylls and sporophylls in fern i.e. every leaf of fern forms sporangia at the time of reproduction. Therefore cones are not produced in ferns. E.g. Azolla, Adiantum, Alsophila, Pteridium, Dryopteris, Marsilea, Pteris and Salvinia.
Pteridophyta is divided into 4 classes :
(1) Psilopsida :
(2) Lycopsida :
Club mosses are placed in this class. The plant body of club mosses is differentiated into root, stem and leaves. Leaves are green and photosynthetic and known as tropophylls. Sporophylls are present in group at the tip of the plant. This group is called strobilus and cone. E.g. Lycopodium and Selaginella.
(3) Sphenopsida :
In this class, Horsetails are included. The plant body of horsetails is differentiated into root, stem or rhizome and scaly leaves. Silica is present in the epidermis of these plants. Due to this, the surface of the plant becomes rough. If two horsetail plants collide, then there is a dangerous chance of fire in the forest. The formation of sporangia takes place on special branches of stem called sporangiophores. Sporangiophores are arranged in a group and form a long and condensed cone. This cone is formed at the apical part of the aerial stem. E.g. Equisetum.
(4) Pteropsida :
This is the largest group of pteridophytes. They are commonly called as ferns. Most of the pteridophytes are ferns. Ferns are megaphyllous i.e. rhizome is small and leaves are comparatively larger and these leaves are known as 'Fronds'. Multicellular hair is present on the young leaf and young rhizome of ferns which are called ramenta. They are for protection. There is no difference between tropophylls and sporophylls in fern i.e. every leaf of fern forms sporangia at the time of reproduction. Therefore cones are not produced in ferns. E.g. Azolla, Adiantum, Alsophila, Pteridium, Dryopteris, Marsilea, Pteris and Salvinia.
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