THE THALLOPHYTES 349 
unicellular, they frequently are united in colonies. They all 
possess chromatophores containing chlorophyll but this green 
pigment is often obscured by the presence also of a brown 
pigment. 
The most striking peculiarity of the group is the structure of 
the enclosing cell wall. This is in the form of a siliceous case 
consisting of two valves which fit into each other like the halves of 
Nodule — fe: 
ia 
mia 
ett 
Nodule - 
Orn 
Et =" Nucleus 
Fic. 253.—Two views of the diatom Pinnularia. A, valve view; B, girdle view, 
showing cell contents. (After Strasburger’s Text Book of Botany, Macmillan Co. 
publishers.) 
a pill box. The valves, which are beautifully sculptured with 
intricate patterns of lacework or of beading, are similar except 
that one is slightly larger than the other so as to fit over it. 
Diatoms vary in form, being either circular, linear, elliptical, 
cylindrical, rhomboidal, triangular, stellate, wedge-shaped, or 
fan-shaped, etc. Some are borne on the ends of stalks, while 
others are held in gelatinous masses. Their siliceous skeleta 
are deposited constantly on the floor of ponds, rivers, lakes and 
