Chlorophyceae 171 



abundantly covered with Coleochaete, imbed in paraffin, and cut 

 host and guest together. 



Diatoms. Living diatoms are often found clinging in great 

 numbers to filamentous algae, or forming gelatinous masses on vari- 

 ous submerged plants. Cladophora is frequently covered with 

 Cocconeis, an elliptically shaped diatom; Vaucheria is often covered 

 with small forms. Other algae will pay for examination, especially 

 if they look brown. If stones in the water have a brown, slippery 

 coating, you can be sure of diatoms. Sometimes the brown coat- 

 ing on sticks and stones is so abundant that it streams out with the 

 current. If rushes and stems of water plants have a brown, gelati- 

 nous coating, you are likely to find millions of specimens of the same 

 diatom. The surface mud of a pond, ditch, or lagoon will always 

 yield some diatoms. They can be made to come out from the mud 

 by putting a black paper around the jar and letting direct sunlight 

 fall upon the surface of the water. The diatoms, in a day or even 

 less, will come to the top in a scum which can be easily secured. 



Fresh-water diatoms appear in greatest abundance in spring, 

 are comparatively scarce in summer, and reappear in autumn, 

 though not so abundantly as in the spring. 



Marine forms can be secured by scraping barnacles, oyster shells, 

 and other shells. The big Strombus shell from the West Indies, which 

 we use to keep the door open, will yield a good collection if you get 

 it before it is cleaned. 



The silicious shells of diatoms are among the most beautiful 

 objects which could be examined with the microscope (Fig. 36). 

 To obtain perfectly clean mounts requires considerable time and 

 patience, but when the material is once cleaned, preparations may 

 be made at any time with very little trouble. Diatom enthusiasts 

 have devised numerous methods for cleaning them, and separating 

 the various forms from each other, but we shall give here only a few 

 simple, practical methods. 



Material for mounts of frustules of living forms "may be obtained 

 by skimming off the brownish scum found on ponds, by squeezing 

 out water weeds, by scraping sticks and stones which are covered at 

 high water, or from the mud of filter beds and pumping-works, or 



