DESMIDS, DIATOMS, AND FRESH-WATER ALG^K. 63 



The desmids float freely in the water; many diatoms 

 do the same. Several species of desmids are attached 

 to one another side by side to form long bands; many 

 diatoms are arranged in a similar way. Some des- 

 mids are surrounded by a colorless jelly-like envelope; 

 so are some diatoms. The desmids never grow on 

 the ends of stems secreted by themselves, and at- 

 tached to other plants or to submerged objects; many 

 diatoms are found growing on the extremities of long 

 colorless and- branching stalks like microscopic trees, 

 these stems being supported by other objects in the 

 water. Some of the commonest diatoms will be found 

 in great abundance growing in this way on the leaves 

 of Myriophyllum. Any object that may apparently 

 be either a desmid or a diatom is not a desmid if it is 

 on the end of a stem of its own formation. 



Most desmids have the ability voluntarily to change 

 their position. They can move from place to place, as 

 they frequently do when under the microscope, slowly 

 travelling across the field of view in an exceedingly in- 

 teresting way. When mixed with mud or with other 

 extraneous matters, as' they often are when gathered 

 and carried home in a bottle, they will gradually work 

 themselves to the surface and- collect in a green film 

 or line on the side of the vessel next the window, 

 whence they can be easily taken by the dipping-tube. 

 Diatoms have a similar power of movement; but they 

 are usually much more active, and their motions more 

 rapid than those of desmids. And while the desmids 

 rnove stately and slowly in one direction, a diatom 

 may travel quickly half-way across the field of view, 

 and without a moment's hesitation, and without turning 

 around, may at once return by its former path or dart 



