216 



GENERAL MORPHOLOGY OF PLANTS 



the cell wall, as shown in fig. 170. It often takes place so regu- 

 larly that the contents of the cell are collected into a well-defined 

 sphere or elliptical body inside the cell. Now if the salt solution 

 be removed and fresh water added, the movement of water will 

 be inward again, the cell will recover from the state of plasmolysis 

 and be restored to the state of turgor. 



353. The growth and multiplication of spirogyra. The 

 thread of spirogyra grows by the 

 division of cells and then elongation 

 of the cells. The nucleus divides first 

 into two nuclei and a cross wall is 

 then laid down between them. The 

 two daughter cells are at first shorter 

 than the parent cell, but each one 

 soon elongates. This process taking 

 place in all of the cells brings about 

 the rapid elongation of the thread. 

 Multiplication of the threads takes 

 place by the separation of a single 

 thread into several shorter ones, the 

 thread breaking at a cross wall, by 

 the splitting of the cross wall, so that 

 the cell is not injured. Strong inside 

 pressure, which sometimes results by 

 a change in the water, often causes 

 this separation of the threads. 



354. Reproduction by conjuga- 

 tion. Sexual reproduction in spiro- 

 gyra takes place by a process known 

 as conjugation. When the conditions 

 are favorable two threads lying near 

 each other conjugate by tubes devel- 

 oped from opposite cells. The tubes 

 meet and the walls at the point of 

 contact dissolve, making an open 

 communication between the two cells. This is the conjugation 



U _ 



Fig. 172. 

 Zygospores of spirogyra. 



