34 THE MICROSCOPE. March 



with a quick or jerky motion. A form may go half across 

 the field in one direction, and then suddenly start off in 

 another. Diatoms are found in both fresh and salt water ; 

 in fact, in almost any depression which contains water. 

 The cell-wall is hard and brittle and is broken with diffi- 

 culty, while the edges are sharp and clear cut. A cell 

 may break into several pieces but without showing 

 ragged edges. Diatoms are not only found alive, but 

 many of them are fossil. They can be heated red hot or 

 boiled in acids and still retain their beautiful markings. 

 Indeed, in order to study them thoroughly and intelli- 

 gently, they should be so treated. Not only this, but 

 it takes the highest powers of magnification, the nicest 

 apparatus, and the most skillful manipulation to bring 

 out these features and to reveal their true structure. 

 Pleurosigma angulatum, a salt water form, is often used 

 to test the power of lenses. But this need not deter us 

 from learning their forms and some of their peculiarities. 

 We can find enough in this to interest ourselves and our 

 friends for many evenings. 



A diatom is formed much like a pill-box, it consists of 

 two parts or valves, one corresponding to the box, the 

 other to the cover, and they fit one over the other in much 

 the same way. In looking tit them, sometimes the front 

 of the diatom is seen. This corresponds to the edge of 

 the pill-box, and then it will have quite a different ap- 

 pearance from that presented when the top or bottom is 

 shown. By tapping the cover-glass the diatom will usu- 

 ally roll over. Diatoms sometimes secrete a stem of 

 their own, a desmid never does this. The diatoms grow 

 on the ends of the stems, which branch and look like 

 microscopic trees. These trees attach themselves to 

 other plants. A good way to collect them is to take a 

 small bit of some fine-leaved aquatic plant and with the 

 side of the dissecting needle gently scrape the leaves. 



