The human naked eye can differentiate between two points, which are at least 1.0mm apart. This is known as resolution of eye. The resolution can   be increased with the aid of lenses. In a typical compound microscope the resolution is 2.0um, which is about 500X that of naked eye. A compound microscope is a typical laboratory microscope with at least different magnification power power. The typical laboratory microscope with at least different magnification powers.

The typical ocular lenses could be 5X and 10X but others also exist. Likewise different type of objective lenses viz. 20X, 40X, 100X etc exist. The magnification power of microscope is determined by multiplying X values of the objective lenses. Therefore, a microscope with 10X ocular lens ans 40X objective lens will have (10X40+400X) 400X magnification power. The resolution will, however, remain the same, which is 500X that of the naked eye. The source of illumination is such microscope is visible light. In electron microscope rages between 2-4 Angstrom, which make it 500X greater than that of the compound microscope and 250,000X greater than that of the naked eye. This means that two points which are 2-4 Angstrom apart can be differentiated with the help of electron microscope. The revelation of complexity of structure of various cellular organelles is closely linked with the development of microscopy and improvement in the resolution power of the microscope.

The salient features of cell theory in this present from are:

  1. All organisms are composed of one or more cells.
  2. All cells arise from pre-existing cells.
  3. Cell is the basic structural as well as functional unit for all organisms.
  4. Cell as a unit of structure and function

A cell is a unit of structure ans function in living organisms. In multicellular organisms there is a division of labor among cells. Different cell are specialized fore different functions. The function of the organism as a whole is the result of the sum of axtiviries ans interactions of different cells ans of different components of the cell. In animals e.g., muscle cell contract and relax, nerve cells transmit impulses, gland cells secrete, red blood cell carry oxygen and some stomach cells secrete gastric juice similarly in plants xylem cell conduct water and mineral salts from soil to the aerial parts of the plants.

Phloem cells translocate food, sclerenchymatous, cell give support to surplus food and meristematic cells produce new cell for growth and development of the plant. As they perform different functions they show great variation in shape and size. Despite the structural and functional diversity, the plant cells as well as animal cells have a common plan of organization.