XI.] SPIROGYRA. 399 



7 The nucleus contains a relatively large nucleolus, the 

 substance of which seems to be homogeneous, and some- 

 what resembles that of the pyrenoids in its reactions. ; The 

 nucleus itself usually appears to be finely granular. Its real 

 structure is however fibrous, but for the study of these 

 details the nucleus of Spirogyra is not favourable. 

 (The growth of Spirogyra in length is accompanied by the 

 increase in number of its cells. This increase takes place 

 by the repeated division of each cell into two equal parts, 

 by means of a transverse wall; the process is not localized 

 in any special region of the filament; all its. cells are alike 

 capable of division. , In this fact Spirogyra differs from all 

 the more perfect plants to be subsequently considered, and 

 here also we have an indication of the physiological equiva- 

 lence of the constituent cells of Spirogyra.) 



The process of cell- division takes place, under normal 

 conditions, in the night, the products of assimilation which 

 have been accumulated in the daytime being used up during 

 the darkness to form new cell-wall and protoplasm.j (The 

 division is however dependent on temperature, and by keep- 

 ing the plants cold during the night it is possible to make 

 them postpone their cell-division till the morning, when it 

 can be conveniently observed.} [The nucleus divides into 

 twb, thedaughter nuclei being at first connected by fine 

 protoplasmic threads.Vln the meantime a transverse ring of 

 cellulose is formed by the protoplasm around the cell, mid- 



1 7" . 



way between its two ends.) f This ring is continuous, at its 

 outer edge, with the lateral wall on which it abuts. By 

 further additions from the protoplasm it gradually grows in- 

 wards until its inner edges meet, so that a complete disc of 

 cellulose is formed across the cavity of the_cell.J (The con- 

 tents necessarily become constricted by the inward growth 

 of the new cell-wall, and are eventually severed into two 



