BOTANY 



The cells are either solitary or form colonies; they are free- 

 floating, or are attached by means of gelatinous stalks, excreted by 

 the cells themselves (Fig. 319). Sometimes the cells remain con- 

 nected and form bands or zigzag chains, 

 or, on the other hand, they are attached 

 and enclosed in gelatinous tubes, while 

 in the case of the marine genus Scliizo- 

 nema they lie embedded in large numbers 

 in a gelatinous branching thallus. The 

 cells also display a great diversity of 

 shape ; while generally bilaterally sym- 

 metrical, they may be circular or ellip- 

 tical, rod- or wedge-shaped, curved or 

 straight. The structure of their cell 

 walls composed of pectic substance that 

 is silicified is especially characteristic ; 

 it is formed of two halves or VALVES, 

 one of which overlaps the other like 



FIG. 319. Licmophora flabellata. 

 Colony of Diatoms with branched 

 gelatinous stalks. (After SMITH, 

 from GOEBEL'S Organographie.) 



FIG. 320. Planktoniella sol. Atlantic Ocean. A disc- 

 shaped plankton diatom with a hollow floating wing 

 arising from the girdle side. The protoplast con- 

 tains a nucleus and numerous chromatophores. 

 (x 322. After G. KARSTKN.) . 



the lid of a box. The cells thus present two altogether different 

 views, according to the position in which they are observed, whether 

 from the GIRDLE or VALVE SIDE (Fig. 79). 



The lateral walls of the two valves are formed of the girdle pieces attached 

 beneath the margins. In some genera the girdle side is extended by the intro- 

 duction of annular or scale-shaped intermediate bands. 



