SPIROGYRA 209 



is suspended by long thin bridles (br.), each of which 

 runs into a chloroplast opposite a pyrenoid (p). This 

 is probably connected with the function of the nucleus 

 in controlling nutrition (see p. 67), since it is round the 

 pyrenoids that starch is laid down. 



The cells of Spirogyra divide, after karyokinetic 

 division of the nucleus, exclusively in the plane per- 

 pendicular to the long axis and halfway between the 

 end walls, the new cell wall, secreted and constantly 

 covered by cytoplasm, growing out in the form of a 

 ring from the cylindrical wall, the effect being like 

 the gradual closing of an iris diaphragm, till the two 

 daughter cell cavities are completely separated. The 

 cells then grow in length till they reach the length of 

 the standard cell of the species. Under certain con- 

 ditions the thread breaks up into lengths, or even into 

 single cells, by the splitting apart of adjacent cells ; 

 and in this way the number of individual threads 

 increases. Spirogyra is sometimes said to be " physio- 

 logically unicellular," because each cell functions as 

 a self-contained unit — ^it is immaterial to its life 

 whether it is isolated or whether it forms part of a 

 thread. 



Gamete Formation and Conjugation. — Under certain 

 conditions two threads lying side by side form gametes, 

 one from each cell. The cell walls of one of the threads 

 on the side towards the other thread are thrust out 

 in blunt projections, one from each cell, and the cell 

 body of each cell begins to leave the wall. Almost 

 immediately similar projections are thrust out from 

 the cells of the other thread opposite those, of the 

 first set, the pairs of projections meet between the 

 threads, the parts of the cell walls in contact are 

 absorbed, and an open conjugation canal is thus formed 



14 



