THE CELL. 39 



forcing water through stoppers of wood which consisted mainly of 

 tracheids by employing considerable pressure (3-4 atmospheres), 

 and the quantity of water pressed through was proportional to the 

 pressure. But according to PAPPENHEIM'S more recent investi- 

 gations ' and Russow's earlier observations (1877) a pressure of one 

 atmosphere suffices to force the torus against the pore opening, and 

 the amount of water passed through is thereby reduced (Pappenheim). 

 GODLEWSKY supposes the torus to function similar to a platinum 

 cone used in the chemical laboratory. According to this investi- 

 gator the margin of the torus is crenated, and fine radial thickenings 

 or something similar are supposed to extend from the margin of the 

 torus over the margin of the closing membrane (margo). In this 

 way the margo when pressed against the pit cavity acts similarly to 

 a folded filter ; the crenated margin of the torus prevents the tear- 

 ing of the filter, and still permits the passage of water. 3 



Bordered pits are therefore all those thin cell-wall areas of the 

 above-mentioned secondary vessels ; consequently not only of porous 

 vessels but also of reticular and scalariform vessels. Accordingly a 

 cross-section or profile view of two contiguous cell-walls of a 

 scalariform vessel, for example, would appear as shown in Fig. 18. A 

 surface view of the same pits is represented at the right in the figure. 



We will now pass to the localized surface-growth of the cell- 

 wall. We have already become familiar with one instance of 

 the elongation of annular and 

 spiral vessels. Another in- 

 stance is to be observed in 

 cell-division (rather the prep- 

 aration to divide) of the alga 

 group Desmidiacece. Their 

 asexual development takes 

 place through division. Fig. 

 19 explains itself. We are 

 here concerned with the zone 



U- U V - ^ F 10 - 18 ' 



which lies in the region of 



the constriction of the cell ; it is the growing zone. After the 



elongation of this zone a septum is formed in its middle portion. 



1 Berichte der deutschen botauischen Gesellschaft, 1889. 



2 Russow's communication " Zur Kenntuis des Holzes, etc.," is to be found in 

 Bot. Centmlblait, XIII; GODLEWSKY'S communication iu Pringsheim's Jahr- 

 bilcher fur wissenschaftliche Bot., XV (1884). 



