It will be shown afterwards that the primordial cell sometimes constitutes the whole plant, with or without its cellular coat. By its continual bisection when so coated, linear plants, such as the confervæ, are formed and lengthened (fig. 3). When bisection is about to take place, the cell increases in length; the nucleus, which always plays an important part in cell formation, spontaneously divides into halves; at the same time the cell wall becomes constricted in the middle and gradually folds between them, and divides the original cell into two new ones, in which the nuclei become perfect and assume their normal position. The terminal cell may undergo the same process, so that the plant may be lengthened indefinitely. [Illustration: Fig. 3. Development of Ulva:—A, isolated cells; B, C, clustered subdivided cells; D, E, confervoid filaments; F, G, frond-like expansions.] Plants which spread in two directions are formed and increased by the successive division of the cells into four equal parts, as in some of the fuci, and the solid vegetable mass is formed and augmented upon the same principle, so that it consists of a congeries of primordial cells or globules coated with cellulose, which by mutual pressure take a many-sided cellular form. Six or eight sides are most common; when six-sided, a section of the solid is like honeycomb, but it frequently resembles a very irregular fine lace or network. The form of the cells, which not only depends upon the number of sides but on the direction of the pressure, varies exceedingly in different plants, and in different parts of the same plant. The size of the cells averages from the three to the five hundredth part of an inch in diameter. Some are very large, as in the pulp of the orange and lemon; but in the pollen of flowering plants, and other cases, they are not more than the thousandth part of an inch in diameter, consequently invisible to the naked eye. Occasionally the cells are elongated in the direction of least pressure, as in the stems and hairs of plants, or sometimes they have a stellar form. In the looser and fleshy parts they retain their globular form and only touch one another, leaving triangular spaces between them filled with air in water plants; but in general the cells are held together by a viscid liquid. When these intercellular spaces, whether left by globular or polyangular cells, are placed the one over the other for some distance, they constitute intercellular passages or channels, and sometimes they form lacunæ or large empty spaces.