and Laboratory Methods. 1853 



4. Note — In the form following, a return will be made to a typical plant 

 related to Pleurococcus. 



X. Scenadesmiis quadricauda (Turp.) Bre'b. Order, Pleurococcales. Family, 



Pleurococcacese. 



Scenadesmus is very widely distributed, and may be found in the sediment 

 in the bottom of ponds, creeks, etc., along with diatoms and other microscopic 

 plants. It usually consists of a colony of four, more or less spindle-shaped, 

 green cells. The two outer cells have four slender, pointed, prong-like projec- 

 tions extending diagonally outward, one at each corner of the colony. 



1. Mount some of the sediment containing Scenadesmus, and examine 

 under high power. Draw and describe. 



2. Compare a number of colonies as to size, shape of cells, and appearance 

 of the projections. 



ALGAE WITH FANTASTIC CELL WALLS OR WITH COMPLICATED 

 CHROMATOPHORES. 



XI. Diatoms. Order, Diatomales. Family, Diatomaceae. 



This family contains a large number of genera and species both living and 

 fossil. Diatoms can always be found forming brown scums or sediments on the 

 bottom of ponds, creeks, ditches, etc. 



1. Mount some sediment or water containing diatoms and study the different 

 species present. 



2. Under high power, draw six different species, representing them from two 

 to four inches long. They are unicellular plants with two silicified valves or 

 shells which fit together like the lids of a pill-box. Represent carefully the mark- 

 ings on the shell. In some species the ends and central portion of the valves 

 are marked by nodules and these points are connected by a rib or suture called 

 the raphe. These can be seen from the valve view. 



3. Notice the greenish, yellow or brown chromatophores, the nucleus, and 

 the cytoplasm. How are the cell organs arranged ? 



4. Look for chains or filaments of diatoms, also for stalked forms. 



5. Study dividing forms. Some species conjugate. Look for such forms. 



6. Study the movement. Does it have any relation to the field of the micro- 

 scope, or the intensity of the light in the field .'' Describe. What is the cause of 

 the motion ? Remember that the motion is magnified under the microscope. 

 How long does it take a diatom to pass across the diameter of the field ? 



7. Isthmia. Scrape specimens of Isthmia from dry, red or brown algae or 

 study from mounted slides. Isthmia can usually be obtained from dry algae col- 

 lected on the California coast. Draw a specimen from the girdle view, showing 

 the valves and details of the workings. Notice that the individuals are of very 

 different sizes. Draw one showing the valve view. Draw an individual in 

 process of division. Describe how the valves fit together, how new valves are 

 formed, and what is the character of the valves of the two individuals resulting 

 from a division. Explain the cause of the difference in size. 



8. Fossil diatoms. Study material from the Tertiary deposit of Richmond, 



