90 PRACTICAL BOTANY. 



study, recommends that the Spirogyra be placed in a 

 flat plate upon a block of ice during the night : if on 

 the following morning the plants be exposed to a higher 

 temperature, the cell-division which had been pre- 

 viously retarded will begin, and the successive stages 

 may be followed. 



Though the main points in the process of division may be 

 observed in fresh material, the minute details will be better seen 

 in material which has been fixed in 1 per cent, chromic acid for 

 about four hours, or in picric acid : after washing carefully, the 

 material may be stained with carmine or haematoxylin, and 

 mounted in very weak glycerine. 



The following points in the process should be specially 

 noted 



1. The disappearance of the nucleolus. 



2. The formation of the striated nuclear spindle and 

 of the equatorial nuclear disk. 



3. Division of this, and collection of the two halves 

 at the poles of the spindle as the new nuclei, which are 

 still connected by fine threads. 



4. The subsequent dilatation of the spindle, the 

 threads becoming more curved. 



5. Meanwhile the collection of microsomata at the 

 equator of the dividing cell. 



6. The involution of the chromatophores and their 

 subsequent division. 



7. The gradual formation of the septum, beginning 

 at the periphery and proceeding towards the centre. 



III. In summer or autumn the process of conjuga- 

 tion and formation of the zygospores may frequently be 

 observed in Spirogyra : filaments which are about to 



