DESMIDS, DIATOMS, AND FRESH-WATER ALG^. 65 



toms, although many may appear to be so with a low- 

 power lens; but the splendid glasses of the best Amer- 

 ican makers will compel any diatom to show how it is 

 marked and roughened. 



In each extremity of many desmids, especially in 

 the crescent-shaped ones, is a small colorless, appa- 

 rently circular space containing numerous minute 

 black particles in incessant motion. These little 

 granules, which are said to be crystals, are sometimes 

 so few that they can be counted if sufficiently magni- 

 fied, while in other individuals they appear to be innu- 

 merable. Their motion resembles the swarming of 

 microscopic bees. It can scarcely be described, but 

 once seen it can never be forgotten. The spaces con- 

 taining them are called vacuoles, and are never pres- 

 ent in diatoms. It is true that in some of the latter, 

 when dying or dead, many minute black particles are 

 visible, dancing and swarming in clusters within the 

 cells, but this "Brownian movement" is common to 

 many microscopic creatures after death. 



In the desmids there is also often seen a circulation 

 of the protoplasm similar to the cycl6sis in the leaf- 

 cells of Anacharis, a movement of the cell contents 

 never observed, so far as I am aware, in any diatom. 

 Between the cell-wall and the green coloring matter, 

 the chlorophyl, there seems to be a narrow space filled 

 with colorless protoplasm, and it is here that the cir- 

 culation takes place. It is a steady, rather rapid flow, 

 several currents streaming lengthwise up and down 

 the cell, carrying the minute starch grains and other 

 enclosed particles in their course. It has been said 

 that these currents sometimes enter the vacuoles, and 

 that the latter obtain their supply of swarming gran- 



6 



