Spirogyresd 349 



at a later stage then both nuclei may be in one cell the other being again entirely 

 without 1 . 



There is usually one large nucleolus (fig. 217 A), but two, or even three 

 may sometimes occur. The nucleolus contains much chromatin. 



Any cell of the filament may undergo division and from the time Stras- 

 burger ('82) first gave an account of the nuclear division, many investigators 

 have studied the mitosis in Spirogyra. Considerable differences of opinion 

 exist with regard to the actual details and especially as to the number of 

 chromosomes which arise from the nucleolus, Van Wisselingh ('00) stating 

 that only two of them arise in that way, whereas Berghs ('06) and Moll ('08) 

 state that all the chromosomes arise from the nucleolus 2 . The spindle is 

 broad and cylindrical and the balance of evidence is in favour of its purely 

 cytoplasmic origin 3 . 



Amitotic division of the nucleus has been induced by Gerassimoft' ('00), and also by 

 Nathansohn ('00) by subjecting the filaments to anaesthetics, such as chloroform and ether, 

 of a strength of 0'5 1 per cent, in the culture medium. Gerassimoff also obtained the 

 same result by cooling the filaments below freezing-point for a time. These results should 

 be regarded, however, as pathological. 



On the division of binucleated cells the daughter-cells are again binucleated through 

 several generations. 



In the division of the cell the new transverse wall arises as a delicate 

 annular ingrowth from the lateral walls and by gradual extension finally 

 becomes a complete partition. As the new wall gradually thickens the 

 original partition becomes the middle lamella. 



Vegetative propagation sometimes occurs by the fragmentation of the 

 filaments either into individual cells or short chains of cells, which then form 

 new filaments by cell-division. It is in the slender species of Spirogyra, and 

 more especially those in which the cells possess ' replicate extremities,' that 

 this method of multiplication is most frequently observed. The ' replicate 

 extremity' consists of an annular infolding of the end-wall (fig. 217 B), which 

 facilitates fragmentation by becoming everted (fig. 217 C). 



Reproduction occurs by the conjugation of isogametes which always fuse 

 in one of the gametangia. In the formation of a gamete the protoplast 

 contracts away from the wall, receding from the end-walls first. It soon 

 becomes more or less ellipsoidal, after which one gamete of each conjugating 

 pair begins very gradually to glide from its mother-cell (the male gametangium) 

 through the conjugation-canal into the opposite cell (the female gametangium). 



1 The cells without nuclei may live for some weeks and are able to store starch. 



2 For a brief but comprehensive summary of the various views on nuclear division in Spirogyra 

 consult Lutman ('11). 



3 It is probable that the discrepancies in the various accounts of nuclear division in Spirogyra 

 are to be attributed to defective methods of fixation and the consequent misinterpretation 

 of facts. 



