. Textbook of botany. Botany. 46 TEXTBOOK OF BOTANY the outer wall of the cell; finally, when the new wall has grown clear across the cell, the two daughter cells are fully formed, each having all the parts that the mother cell had, but being only half as large. In examining a Spirogyra plant, one always finds that the cells are of different lengths; and often there are two short cells side by side, each about half the length of the longer ceils of the thread. Such a pair of short cells is the result of a recent division, probably one that occurred during the previous night. After a, division
. Textbook of botany. Botany. 46 TEXTBOOK OF BOTANY the outer wall of the cell; finally, when the new wall has grown clear across the cell, the two daughter cells are fully formed, each having all the parts that the mother cell had, but being only half as large. In examining a Spirogyra plant, one always finds that the cells are of different lengths; and often there are two short cells side by side, each about half the length of the longer ceils of the thread. Such a pair of short cells is the result of a recent division, probably one that occurred during the previous night. After a, division the daughter cells grow, and when they have reached their full size each of them again divides. It is by a series of alternate periods of growth and division that the number of cells in a thread increases, and that the thread as a whole grows longer. Cell division is a method of reproduction because it increases the number of cells; it does not, however, at least directly, increase the number of plants. Sometimes a plant is accidentally broken into two or more; and the plants of some species of Spirogyra have a definite method of breaking up at times into single cells or groups of cells. Such a process of breaking up is also one of reproduction which in- creases the number of plants, but not the number of cells. 70. Conjugation. — After the Spirogyra plants have been growing for a time, their cells cease to divide and prepare instead for a very. Fig. i6. ^—Conjugation in Spirogyra. A, the commoner method, by which gametes from different plants unite to form a zygote; B, the conjugation of gametes produced by the same plant. different process, in which two cells are to unite and form a single cell. The first step in preparation for this conjugation is an arrange- ment of the plants so that two lie parallel and near together. Then many or all of the cells of each plant put out projections, one from each cell (Fig. i6. A), which grow toward the cells of the neighboring plant.
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