94 



MORPHOLOGY. 



Tf'm 



plished in many cases without any injury to the cells. In this manner the 

 threads or plants of spirogyra, if we choose to call a thread a 

 plant, multiply, or increase. In this breaking of a thread the 

 cell wall which separates any two cells splits. If we should 

 examine several species of spirogyra we would probably find 

 threads which present two types as regards the character of 

 the walls at the ends of the cells. In fig. 85 we see that the 

 ends are plain, that is, the cross walls are all straight. But 

 in some other species the inner wall of the cells presents a 

 peculiar a.ppearance. This inner wall at the end of the 

 cell is at first straight across. But it soon becomes folded 

 back into the interior of its cell, just as the end of an 

 empty glove finger may be pushed in. Then the infolded 

 end is pushed partly out again, so that a peculiar figure is 

 the result. 



222. How some ot the threads break. — In the separation 

 of the cells of a thread this peculiarity is often of advan- 

 tage to the plant. The cell-sap within the protoplasmic 

 membrane absorbs water and the pressure pushes on the 

 ends of the infolded cell walls. The inner wall being so 

 much longer than the outer wall, a pull is exerted on the 

 latter at the junction of the cells. Being weaker at this 

 point the outer wall is ruptured. The turgidity of the two 

 cells causes these infolded inner walls to push out suddenly 

 as the outer wall is ruptured, and the thread is snapped 

 apart as quickly as a pipe-stem may be broken. 



223. Conjugation of spirogyra.— Under cer- 

 tain conditions, when vegetative growth and 

 multiplication cease, a process of reproduction 

 takes place which is of a kind termed sexual repro- 

 duction. If we select mats of spirogyra which 

 have lost their deep green color, we are likely to 

 find different stages of this sexual process, which 

 in the case of spirogyra and related plants is called 

 conjugation. A few threads of such a mat we 

 should examine with the microscope. If the 

 material is in the right condition we see in certain 

 of the cells an oval or elliptical body. If we 

 note carefully the cells in which these oval bodies 



are situated, there will be seen a tube at one side which con- 



Fig. 85 



iread of 

 gyra, showing lonj 



Thread of spiro" 



„, . i long 



cells, chlorophyll 



band, nucleus, 

 strands of proto- 

 plasm, and the 

 granular wall layer 

 of protoplasm. 



