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



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, but there are other 

 points that can 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 broken, 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 brit- 

 tle. 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 broken, 

 however ; and it does so as if made of glass or some other 

 hard and brittle material, and the yellowish-brown con- 

 tents may flow out, but the broken place will not be a 

 hole with irregular edges, as it was in the crushed des- 

 mid ; the edges will be sharp and angular, and the dia- 

 tom will probably break into several fragments. Yet 

 with the most skilful manipulation it is rather difficult 

 to purposely break any but the largest of the diatoms, 

 few of which are visible to the unaided sight of the 

 acutest eye. The little hard-coated 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, 



