Essential structures in a plant cell

The essential structures comprised in a plant cell may be summarized as follows: -
I. The Cell Wall, basically composed of cellulose but often chemically ered by the incorporation of other materials. This encloses a space, the Cell Lumen, in which the following occur.
2. The Cytoplasm, which includes all the protoplasm outside the nucleus, with certain differentiated
structures :-
The Plasma Membrane, an external surface layer in contact with the wall.
The Vacuolar Membrane or Tonoplast, bounding the vacuoles.
Embedded in the cytoplasm are the following structures :-
(a) The Plastids, which are protoplasmic bodies, denser than the cytoplasm and not separated from it by any definite membrane. They comprise :-
Chloroplasts, the bearers of the green pigment, Chlorophyll. Chromoplasts, \yith colours other than green (the term is sometimes used to include all coloured plastids).
Leucoplasts, which are colourless and found chiefly in underground organs and in the meristem cells.
(b) The Vacuoles, sac-like enclosures in the cytoplasm, filled with a liquid" cell-sap."
(c) The Mitochondria or Chondriosomes, granules, rods or threads apparently composed of phospholipins and proteins, which are found scattered through the cytoplasm of all cells.
(d) The Centrosomes, granules associated with nuclear division, which are characteristic of animals, but are found only in a few of the lower plants.
(e) The Ergastic Substances, or materials secreted by the cytoplasm either as food materials or as by-products. Here are included such things as oil-drops, protein grains or crystalloids, starch grains (in the plastids) and crystals of Calcium oxalate.
3. The Nucleus, almost always single in the cell and composed of the following parts ;-
(a) The Nuclear Membrane, separating it from the cytoplasm.
(b) The Chromatin, which is organised as :-
The Nuclear Reticulum, formed of fine threads, which IS characteristic of the non-dividing or " metabolic" nucleus.
The Chromosomes, which are relatively thick rods, formed from the nuclear reticulum during nuclear diyision and con­stant both in number and form. Chromatin is a compound of nucleic acid with basic protein.
(c) The Nuclear Sap, or Karyolymph, which is colourless and fills all the central parts of the nucleus. It may also contain ergastic reserves, e.g., protein crystals.
(d) The Nucleolus, one or more in each nucleus, which is a spherical granule of material, attached to the reticulum and associated with certain chromosomes. It consists of a mixture of protein and lipin, and normally stains differently from the chromatin.
 cell in a higher organism cannot be looked upon as an independent unit. There is a considerable degree of physiological unity pervading all tissues and the cell must be largely controlled by the functioning of the tissues as a whole. Although itself a synthesis of many smaller components, as we haye seen aboye, it plays a part in the synthesis of a still higher unit, the organism, to which it is subordinate. Modern studies no longer treat the
cell as a static object but as a functioning mechanism, and the growth of experimental cytology, including the micro-dissection of cells, has opened roads of the highest promise tmnrds a fuller understanding of life processes.