236 



GENERAL MORPHOLOGY OF PLANTS 



strong sugar solution. This solution being of a greater concen- 

 tration than that of the cell sap, the flow of the water is now in the 

 outward direction through the plasma membrane, and this is 

 pressed inward from the cell wall, as shown in fig. 179. It often 



takes place so regularly 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. 



366. The growth and multi- 

 plication 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 elonga- 

 tion 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. 



367. Reproduction by conjugation. Sexual reproduction 



Fig. 181. 

 Zygospores of spirogyra. 



