94 TYPE STUDIES 



89. Diatoms. Good material is generally present in sediment from sunlit 

 pools siicli as will furnish desmid material. Brown scums or brown slimy 

 coatings are frequently almost pure growths of diatoms. Make a general 

 examination of material and note especially stalked forms and those united 

 into filaments or chains. Search especially for large, boat-shaped types 

 (generally Pinnularia or Navicula) that move to and fro in the water. Study 

 the cell structure of these latter forms under h.p. Draw : 



A. An upper or valve view (elliptical in outline) which shows a suture, 

 raphe, running almost the entire length of the cell, and a nodule at 

 either end and in the middle region. Two brown chromatophores lie 

 along either side of the cell and the nucleus in the center. Note the 

 globules of oil in the cell. Draw an outline on a large scale, showing 

 the markings on the siliceous shell. 



B. A side or girdle view (rectangular ih outline) which in a large cell 

 will show at the ends an overlapping of the two siliceous shells, 

 inclosing the protoplast, which fit together as a cover fits over a box. 

 Illustrate these points and also the position of the chromatophores, 

 oil globules, and nucleus. 



C. Examine some of the polishing powders, such as electro-silicon, for 

 diatoms, making mounts in water or balsam (App. 13). 



90. Vaucheria, the green felt.** Some species are terrestrial, 

 growing on damp earth and common in flowerpots in greenhouses. 

 Others are aquatic, forming heavy, dark green mats on the bot- 

 tom of muddy pools and ditches. Examine the felted growth, 

 its coarseness, feeling, size of filaments, irregular branching. Do 

 the positions of the filaments bear any relation to the direction 

 of light? 



A. Carefully mount some filaments in water. Draw under 

 m.p. Is the diameter of the filament the same throughout 

 any considerable length? Are cross walls present? Is the 

 protoplasm uniformly green? Colorless, root-like branches, 

 rhisoids, may be found. Under h.p. draw : 



1. The end of a filament, showing the arrangement of the 

 protoplasm, the cell wall, the numerous ehloroplasts and 

 glistening globules of oil, the position of a central vacuole 

 running lengthwise of the filament. 



2. Under the highest magnification draw stages in the multi- 

 plication of the ehloroplasts. 



