4 6 



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 cells of the thread. Such a 

 pair of short cells is the result of a receat 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. 16. 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. 16, A), which grow toward the cells of the neighboring 

 plant. Usually a cell in one plant is opposite a cell in the other ; the 

 projections from two opposite cells grow toward each other, come in 



