62 AQUATIC MICROSCOPY FOR BEGINNERS. 



A living desmid is always green; a living diatom is 

 always brown. This difference in color makes it easy 

 to distinguish the two groups of plants, although there 

 are other points that caji be used by even a color-blind 

 student. The cell wall of the desmid — that is, the 

 thin sack which surrounds the soft green contents — is 

 soft and flexible. If the cover-glass is pressed down 

 firmly with a needle the desmid can be flattened or 

 squeezed out of shape, and the cell-wall can often be 

 ruptured, so that the green and colorless mixture of 

 jelly-like matter filling the plant is forced out. 



The cell-wall of a diatom is hard and brittle. The 

 cover-glass may be pressed upon until the glass breaks, 

 yet the diatom will not be flattened nor its shape 

 changed. It may roll over and look quite different in 

 form when viewed in another position, but it will prob- 

 ably roll back and appear as at first. It can be 

 fractured, however; and it breaks as if made of glass 

 or of some other hard and brittle material, and the 

 yellowish-brown contents may flow out, but the broken 

 place will not be an aperture torn with irregular, more 

 or less rounded margins, as it was in the crushed des- 

 mid; the edges will be sharp and angular, and "the 

 diatom will probably break into several fragments. Yet 

 with the most skilful manipulation it is rather difficult 

 purposely to break any but the largest of the diatoms, 

 few of which are visible to the unaided sight of the. 

 acutest eye. The brittle, hard-coated little plants are 

 often found in fragments, but according to the writer's 

 experience they are broken accidentally, either by 

 being piled on top of each other and so crushed by the 

 cover-glass, or by the rough contact with one another 

 when gathered. 



