78 Elementary Biology. 



distance from the free ends of the thread, exists as a layer 

 lying immediately within the cell-wall. From this layer 

 there stretch to the nucleus threads of protoplasm, the 

 nucleus itself being surrounded by a layer of the same 

 substance. The nucleus may, however, be found lying 

 close to the cell-wall, but always separated from it by the 

 layer of protoplasm above referred to. The cell-sap, a 

 watery fluid containing certain organic substances which are 

 probably products of the metabolism of protoplasm, varies 

 in amount according to the age of the cell. Spirogyra, 

 being an unattached floating plant, has neither apex nor 

 base, though the terminal cells have rounded ends, and 

 less cell-sap than other cells. The nucleus contains a 

 nucleolus, and that again is said by some writers to contain 

 a distinctly differentiated endonucleolus. The structure of 

 che spiral band varies with the species of Spirogyra under 

 consideration. In some forms it is single, in others double, 

 but in all cases it possesses more or less irregular margins, 

 , is of a green colour, and contains starch granules, aggregated 

 at definite intervals or scattered irregularly through it. The 

 band itself consists of a protoplasmic basis and chlorophyll, 

 which is in this case diffused in the protoplasmic band, and 

 not in the form of granules as was the case in Protococcus. 



Increase in length is effected by the division of a pre- 

 viously existing cell into two parts. Division takes place 

 only during the night. The process of division is a some- 

 what complicated one, and is accompanied by a series of 

 changes in the nucleus which have been the subject of much 

 recent study and debate. Without entering into detail, it 

 may be sufficient to indicate the chief features in the pro- 

 cess as follows. 



The division of a cell is preceded by division of the 

 nucleus. The process of division of the nucleus is known 

 as karyokinesis. The structure of the nucleus has already 

 been described (p. 30), and it is necessary for the com- 



