The living medium which possesses the marvellous property of being roused into energy by the action of light and heat, and which either forms the whole or the greatest part of every plant, is in its simplest form a minute globe consisting of two colourless transparent concentric cells in the closest contact, yet differing essentially in character and properties. The external one, which is the strongest, is formed of one or more concentric globular layers of cellulose, a substance nearly allied to starch, being a chemical compound of carbon, hydrogen, and oxygen in the proportions of 12, 10, and 10, respectively.[29] It forms the universal framework or skeleton of the vegetable world, but it has no share whatever in the vital functions of vegetation. It only serves as a protection to the globular cell within it, which is called the primordial cell because it is first formed, and because it pre-eminently constitutes the living part, since the whole phenomena of growth and reproduction depend upon it. In its earliest stage the primordial cell is a globular mass of an azotized colourless organizable liquid, called protoplasm, the life blood of vegetation, containing albuminous matter and dextrine or starch-gum. It is sufficiently viscid to maintain its globular form, but its surface becomes slightly consolidated into a delicate soft film. The viscid albuminous liquid within it is mixed with highly coloured semi-transparent particles containing starch; besides cavities or vacuoles full of a watery vegetable sap of highly refractive power are imbedded in it. By degrees the coloured particles become more and more condensed within a globule of mucus, which constitutes the nucleus of the primordial cell. The watery sap in the cavities increases so much as ultimately to fill nearly the whole of the cell at the expense of the viscid protoplasm, which then merely forms a lining to the cell either coloured or hyaline. The primordial cell then secretes and envelopes itself with the strong protecting coats of cellulose already described. On account of its high colour, which is chiefly green, the whole contents of the primordial cell are called the endochrome. The minute globular nucleus contains a liquid of high refractive power, and is coated with a delicate film. Its structure, which is best seen in the hairs and young parts of plants, is not always the same, nor is it always in the centre of the primordial cell, being sometimes attached to the internal cell wall.[30] On the minute but complicated organ, the primordial cell, vegetable life depends.