SPIROGYRA. 



i37 



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

 threads or plants oi sijirogyra, if \\'e choose to call a thread a 

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

 ceil \\"aU which S(,-]"iarates any t^\'o cells splits. If we slmuld 

 examine several species cif spir(jg)'ra we would proliably find 

 threads wdiicli present two types as regards the character of 

 the walls at the ends of the cells. In fig. 128 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 appearance. 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 pusl-jd partly out again, so that a peculiar figure is 

 the result. 



286. How some oi 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 turgidit)" of the two 

 cells causes these infoliled inner walls to push out suddenly 

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

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



287. Conjugation of spirogyra. — Under cer- 

 tain conditions, when vegetative growth and 

 multiplication cease, a process of reproduction 

 takes place wliich is of a kind termed sexual repro- 

 duction. If we select mats of spirogyra which 

 ha\'e lost their deep green color, we are likely to 

 find different stages of this sexual process, which 

 in the case of sjiirogyra and related plants is called 

 conjugation. A few threads of such a mat -sve 

 should examine with the microscope. If the 

 material is in the right condition we see in certain 

 of the cells an o\'al ot elliptical bod)-. It we 

 note carefulh' the < ells in whicli these oval bodies 

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



1 



tig. 12S. 

 Thread of spiro" 

 gA'ra, siiowing long 

 cells, chloropb\n 

 band, nucleus, 

 strands of proto- 

 plasm, and the 

 granular wall layer 

 of protoplasm. 



