76 . ALG.E. 



Desmidiaceee. Each half of a frustule is called a 

 valve, and the line at which these valves meet is 

 called the suture. That side or aspect of the frustule 

 in which the suture lies (fig. 22/) is the front view, 

 and the other aspect of the frustule (fig. 22 s) is the side 

 view. The frustules are mostly four-sided the main 

 breadths of the two opposite valves forming two sides, 

 and the bent margins of the valves, with the back and 

 front of the hoop, forming the two other sides - y so 

 that the view presented by the side of a frustule is the 

 same as that of a single valve. The suture is the line at 

 which the division of the frustules takes place in the 

 formation of new individuals. In this process the 

 cell-contents divide into two parts, as in ordinary endo- 

 genous cell-formation, the two new surfaces thus 

 produced becoming coated with a new portion of cell- 

 wall or valve, so that two frustules now occupy the 

 place of the original one. At the same time a silice- 

 ous band, encircling the frustules at the line of suture, 

 is formed to fill up the interval between the edges of 

 the parent valves; this is the hoop (PI. V. fig. 22 /;. 

 PL VI. fig. 10/), and beneath it lie the two newly 

 formed valves. In many cases I believe that each 

 half-frustule becomes coated with a new entire cell- 

 wall, with its siliceous valves. 



The frustules of the Diatomaceae are constantly 

 undergoing division when in vigorous growth. After 

 the frustules have divided, the new ones either sepa- 

 rate entirely, as is perhaps most commonly the case ; 

 or they remain united, sometimes completely, so as 

 to constitute a filament (PI. V. fig. 23), while at others 

 the frustules cohere only at the angles (PL VI. fig. 

 23), so as to form a zigzag chain. 



In some species, the frustules are attached to foreign 

 bodies by means of a gelatinous cushion (PL V. fig. 5 ; 

 PL VI. fig. 7) j while in others they are situated upon 

 a simple or branched gelatinous stalk (PL V. fig. 17) 

 or stipes (stipes, a stem) . 



