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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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HISTORY OF CLASSIFICATION OF ORGANISMS

In previous decades there are many scientists came who gave different - different classification of organisms. Some of them are discussed here.

(1) Aristotle : 


Aristotle was the father of biology and father of zoology. He was the Greek political philosophers. He classified plants on the basis of morphological character (growth habit) in three groups :
  1. Trees
  2. Shrubs
  3. Herbs

(2) Theophrastus (371 - 287 B.C.) : 



He is known as father of ancient plant taxonomy and father of botany. He was also the Greek political philosophers. Theophrastus wrote book on plants named Historia plantarum. He also proposed the term Annual, Biennial and Perennial. He gave the artificial classification of plants. He classified plant kingdom in to four groups on the basis of growth habit :
  1. Trees
  2. Shrubs
  3. Under shrubs
  4. Herbs

(3) Carolus Linnaeus (1707 - 1778) : 



His real name was Carl Von Linne. On the basis of work in Latin language, he changed his name to Carolus Linnaeus. He was the Swedish scientists. He is known as father of Taxonomy, father of plant taxonomy and father of animal taxonomy. He gave the two kingdom system of classification. He grouped plants and animals into kingdom Plantae and kingdom Animalia respectively.

(4) George Bentham (1800 - 1884) and Joseph Dalton Hooker (1817 - 1911) ; 


Both Bentham and Hooker were related to Royal botanical garden. They wrote the book "Genera plantarum" (1862 - 1883). In this book Bentham and Hooker gave the biggest and natural classification of Spermatophyta i.e. plants with seeds. He classified division Spermatophyta (202 families) into three classes Dicotyledonae (165 families), Gymnospermae (3 families) and Monocotyledonae (34 families). 

The merits of Bentham and Hooker classification was that it was natural classification and the classification of Bentham and Hooker was mainly based on the floral character. This was very appreciable because floral characters are more stable than vegetative (root, stem, leaves) characters. And it is the simplest classification, Although it is not the best classification but yet the arrangement herbarium sheets in herbarium is based on it, because it is the simpler one. The main reason for its simplicity is that this classification is based on actual observations.

The demerits of Bentham and Hooker classification is that in this classification the phylogeny of plants is not considered, because in it, gymnosperms are placed in between dicots and monocots. The sequence of evolution is as follows :  Gymnosperm → Dicots → Monocots

(5) A.W. Eichler : 


He gave the first phylogenetic classification of plant kingdom. The classification of Eichler is very little phylogenetic. He classified plant kingdom into two sub kingdom Cryptogamia (Those plants in which sex organs are not visible) and Phanerogamia (Those plants in which sex organs are visible) and cryptogamia is divided into three divisions Thallophyta, Bryophyta and Pteridophyta and phanerogamia is divided into two divisions Gymnosperm and Angiosperm. In this way Eichler classified plant kingdom into five divisions and arranged them in the order of evolution. 

Thallophyta → Bryophyta → Pteridophyta → Gymnosperm → Angiosperm

(6) Oswald Tippo :

He proposed the biggest phylogenetic classification of plant kingdom. This classification is the complete classification of plant kingdom. This is the most acceptable classification for books and study. In this classification he divide plant kingdom into two sub kingdom on the basis of embryo into Thallophyta (Embryo absent) with 10 divisions and Embryophyta (Embryo present) and Embryophyta is divided into two divisions on the basis of vascular tissue, Atracheata = Bryophyta (Non vascular plants) and Tracheophyta (Vascular plants).

(7) R. H. Whittaker :


R. H. Whittaker in 1969 proposed a five kingdom classification. The kingdoms defined by him were named as Monera, Protista, Fungi (Mycota), Plantae and Animalia. The main criteria used by him for making classification are :
  1. Cell structure (Complexity of cell)
  2. Thallus organisation (complexity of organism) / Body organization.
  3. Mode of nutrition.
  4. Reproduction / Life style
  5. Phylogenetic relationship

He classified all organisms except virus, viroids and lichens into five kingdoms given bellow :
  1. Monera : All the prokaryotes included here (Eubacteria, Rickettsia, Actinomycetes, BGA, Archaebacteria, Mycoplasma).
  2. Protista : All the unicellular Eukaryotes included here (Dinoflagellates, Diatoms, Euglenoids, Slime moulds, and protozoans)
  3. Mycota (Fungi) : True fungi are included here.
  4. Plantae : All the multicellular plants are included here.
  5. Animalia : All the multicellular animals are included here.


(8) Carl Woese : 

He gave the three domain theory. In these three domain 6 kingdoms are included. He suggested the separate kingdom for Archaebacteria. He classified organisms into three domains on the basis of sequence of r-RNA, Archaea, Bacteria and Eukarya and Eukarya is divided into four kingdom Protista, Mycota (Fungi), Plantae, Animalia.

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