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Eyes: How They Work, Anatomy and Common Condition

 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 resp
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BRYOPHYTA

The term Bryophyta was proposed by Robert Braun. The study of Bryophyta is known as Bryology. Father of Bryology is considered to be Hedwig but according to some scientists, it is believed that Cavers is the father of Bryology. Father of Indian Bryology is Professor Shiv Ram Kashyap.

Bryophytes are the first land plant. It is believed that they originated from aquatic plants and they come on land through water because some bryophytes have characters similar to aquatic plants. They are known as amphibians of the plant kingdom because these plants can live in soil but are dependent on water for fertilization. Bryophytes are not considered as the successful land plants because vascular tissue is absent and they need water for fertilization. Due to absence of vascular tissue brayophytes can not grow very tall. The process of water conduction in bryophytes takes place with the help of parenchyma. Roots are absent in bryophytes. Stem like and leaf like structure of bryophytes are functionally similar to the stem and leaves of higher plants. Bryophytes are sciophytes, i.e. bryophytes prefer to grow in moist (wet) and shady places.

Life cycle of bryophytes :

The main plant body of bryophytes is haploid. It produces gametes, hence is called a gametophyte. Sex organs are formed on gametophyte. Sex organs are multicellular and jacketed in bryophytes. Male sex organs are called as antheridium and female sex organs are called as archaegonium. Archaegonium is flask shaped. The male gametes of bryophytes are motile. These motile male gametes are called as antherozoids. Abtherozoids are usually comma shaped and biflagellate. Female gamete is called egg. In bryophyta, fertilization is performed by zoidogamy i.e. male gamete swims into water to reach the female gametes and fertilizes it. As a result of the fertilization, a diploid zygote is formed. Zygote does not undergo reduction division (meiosis) immediately. This zygote initiates the sporophytic generation. Sporophytic generation is a diploid stage. Zygote formed embryo and then sporophyte by mitosis, which is known as sporogonium. Some of cells present in capsule of sporophyte function as spore mother cells. Now meiosis takes place in spore mother cells, result of it haploid spores are formed. The germination of spores is direct or indirect. In liverworts and hornworts the germination of spore is direct i.e. each spore forms a gametophyte after germination i.e. each spore forms one thallus. But the germination of spores in mosses is indirect. In mosses a multicellular filament is formed after the germination of spore. This filament is known as protonema . Now buds are formed on protonema. Each bud develops and form a gametophyte plant. Indirect germination is best for survival. Mosses are gregarious in nature because they appear in group. Sexual reproduction in bryophytes is oogamous type and life cycle is haplodiplontic type. In Bryophyta the sporophyte is depend on gametophyte. This is a unique character of bryophyta.

Bryophyta is divided into three classes :

(1) HEPATICOPSIDA - Liverworts :

Bryophytes included in this class have shape like liver (e.g. Marchantia) or flat (e.g. Riccia) so they are known as liverworts. Plant body of this group is thallus like and dorsiventral. Rhizoids and scales are present on thallus. Rhizoids are unicellular and unbranched. Scales are multicellular. Te leafy members (e.g. Porella) have tiny leaf like appendages in two rows on the stem like structures. The sporophyte of liverworts is completely depend on gametophyte i.e. it is depend on gametophyte for food, water and habitat. The sporophyte of liverworts is made up of foot, seta and capsule. (Except Riccia sporophyte is made up of only capsule). True Elaters are present in sporophyte of some members of liverworts. (e.g. Marchantia). Elaters are hygroscoic and they help in dispersal of spores.
Examples of liverworts : Riccia, Marchantia, Cryptothallus, Porella.

(2) ANTHOCEROTOPSIDA - Hornworts :

The plant body of this group is also thallus like. Scales are absent but rhizoids are present on thallus. Rhizoids are unicellular and unbranched. The sporophyte of hornworts is divided into foot and capsule. The sporophyte of hornworts is not completely depend on its gametophyte i.e. it is semiparasite because its sporophyte is photosynthetic therefore it can manufacture their own food. So it does not depend on gametophyte for food, it depends only for water and habitat. In hornworts at the basal part of capsule, a special, type of meristem is present. Due to the activeness of this meristem, the capsule grows rapidly. It grows like the horn of animals. In hornworts, pseudoelaters are present which help in spores dispersal. Examples of hornworts : Notothylus, Anthoceros.

(3) BRYOPSIDA - Mosses : 

All the mosses are included in this class. The plant body of mosses is made up of stem like, leaf like and rhizoids(root like). The Rhizoids present in the plants of this class are multicellular, branched an obliquely septate. The presence of leaf like structure in gametophyte is the unique character of moss because in plant kingdom any gametophyte do not have leaf like structure. They consists of upright selender axis bearing spirally arranged leaves. Vegetative reproduction in mosses is by fragmentation and budding in the secondary protonema During sexual reproduction, sex organs are produced at the apex of the leafy shoots. The sporophyte in mosses is more elaborated (developed) than that in liverworts. The sporophyte of moss is divided into foot, seta, capsule. The sporophyte of mosses is also semiparasite like, that of Hornworts, i.e. it is photosynthetic. The mosses have an elaborate mechanism of spore dispersal. Peristomial teeth are present in moss sporophyte which help in spores dispersal. Examples of mosses : Funaria, Polytrichum, Buxbaumia, Sphagnum.

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