234 GENERAL MORPHOLOGY OF PLANTS 



often found floating on the water. Some of the cells in Anabczna 

 become much longer than the others and larger. These function 



as spores and multiply the plant. 



377. Review of the blue-green 

 algae. In addition to the characters 

 and habits given under general char- 

 acters it is to be noted that they differ 

 from the other algae by the absence 

 of sexual reproduction, or at least that 

 sexuality has not yet been discovered 

 in the group. 



THE DIATOMS (DIATOME^ : 



BACILLARIALES). 

 A & df C 378 . General characters.^-The 



Fig. i93- ,. A , n j i 



Nostoc linckii. A, filament with diatoms are single-celled plants, 



fso!a[fd e few remaining loosely joined into 

 filaments. They are remarkable for 

 the possession of a silicious skeleton, 



called a frustule, which encloses the protoplast. This skeleton is 

 in two parts (valves) resembling a box with its cover, some of the 

 forms resembling an old-fashioned pill box (fig. 194). Some of the 

 frustules are remarkable for the fine and beautiful sculpturing, and 

 are used for test objects in determining the resolving power of 

 high-grade microscopic lenses. The diatoms occur in fresh and 

 salt water. They exist in vast numbers. The diatoms together 

 with other microscopic plants and animals form what is called the 

 plankton (wandering life of the sea) of fresh and salt waters, 

 especially of the deep waters. Extensive deposits of diatomaceous 

 earth several feet in depth, made up almost entirely of the skeletons 

 of diatoms, exist, for example, in southern England, in places in 

 the Adirondack Mountains, at Richmond, Va., in Nevada, Cali- 

 fornia, etc. This is sometimes used as a polishing powder. The 

 frustules also occur in guano. 



379. The frustules of the diatoms are of various forms, 

 elliptical, oblong, wedge-shaped, circular, etc. Many are free, 



