66 MICROSCOPY FOR BEGINNERS. 



or by the rough contact with one another when gath- 

 ered. 



The desmids float freely in the water ; many diatoms 

 do the same. Several species of desmids are attached 

 to each other side by side to form long bands ; many 

 diatoms are arranged in a similar way. Some desmids 

 are surrounded by a colorless jelly-like coating ; so are 

 some diatoms. The desmids never grow on the ends 

 of stems secreted by themselves, and attached to other 

 plants or submerged objects ; many diatoms are found 

 growing on the extremities of long colorless and branch- 

 ing stalks like microscopic trees, these stems being fast- 

 ened to other objects in the water. Some of the common- 

 est 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 to voluntari- 

 ly change their position. They can move from place 

 to place, as they frequently do when under the micro- 

 scope, slowly travelling across the field of view in a 

 very interesting way. "When mixed with mud or dirt, 

 as they often are when gathered and carried home in a 

 bottle, they will gradually work themselves 'to the sur- 

 face and collect in a green film or line on the side of 

 the bottle 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 des- 



