Desmidiacese 369 



new positions (fig. 231 A). Each nucleus arrives in the notch of the con- 

 stricting chloroplast before that division is completed (fig. 231 B /)). The 

 new cross- wall is formed at right angles to the old walls (fig. 231 A) very 

 much as in Spirogyra. It grows in from the periphery of the cell during the 

 metaphase of nuclear division and cuts across the spindle fairly at its centre, 

 being a third of the way across by the time the fibres have disappeared. The 

 daughter-nuclei have been reconstructed and are moving to their new 

 positions before the new wall is completed. The young semicells are cone- 

 shaped (fig. 231 D), but they soon acquire their proper characters. Lutman 

 states that in these two species of Closterium cell-division and nuclear- 

 division represent at least a two-night process, the chloroplasts dividing the 

 first night and the nucleus probably on the second. 



In Mesotf&nium caldariorum, also an unconstricted Desmid, cell-division is 

 precisely as in the Placoderm Desmids (G. S. W., '15). After the division of 

 the nucleus a new cell-wall is laid down in an exactly transverse plane 

 (fig. 231 F] in a manner very similar to that which occurs in the ZygnemacesB. 

 There is only one chloroplast in the cells of Mesottvnium and the new trans- 

 verse wall cuts right across it. On the completion of this wall there is no 

 trace of a constriction of the cell (fig. 231 G). The new semicells are now 

 developed as in other Desmids. The middle lamella of the new transverse 

 wall gradually disappears from the periphery inwards, and during its dis- 

 appearance, which probably results from its conversion into mucilage, the 

 part of the new wall belonging to each semicell begins to bulge outwards, 

 assuming a greater and greater convexity (fig. 231 H and /). This finally 

 results in the separation of the daughter-cells. 



As a rule all growth ceases in a Desmid after the new semicell is fully 

 formed and has acquired its distinctive specific characters, the cell then con- 

 sisting of two halves of different generations. [Consult the scheme of normal 

 cell-division in Closterium given in fig. 232 /, showing four generations, the 

 semicells of each generation marked s, s^s.,, s s .] In certain species of Penium 

 and Closterium, however, there is a further growth subsequent to completed 

 cell-division. This consists in the development of an elongated median girdle- 

 band, which is a cylindrical piece of cell-wall intercalated between the old 

 and the new semicells. The general scheme of this type of division was first 

 portrayed in detail by Littkemuller ('02) and is diagrammatically represented 

 in fig. 232 //. The scheme depicted is an ideal one, representing the growth 

 of a new girdle-band after each division, but as a matter of fact this rarely 

 happens, and many divisions may take place without the addition of a new 

 girdle-band. In the genus Closterium each division results in a new junction 

 of the old and new cell-walls, the optical effect of which is the addition of a 

 new transverse suture (consult fig. 232 ///and IV). In no known species of 

 Closterium are more than two girdle-bands ever developed, but in Penium 



w. A. 24 



