1698 Journal of Applied Microscopy 



LABORATORY OUTLINES. 



For the Elementary Study of Plant Structures and Functions from 

 the Standpoint of Evolution. 



SERIES I — THALLOPHYTA. A STUDY OF THE LOWEST, 

 NON-SEXUAL FORMS. 



IV. Pleurococcus vulgaris (Menegh.). Order, Pleurococcales. Family, 

 Pleurococcaceae. 



This is a unicellular green alga which is very commonly found forming a 

 green, powdery layer on the bark on the north side of trees, on wooden fences, 

 etc. 



1. Scrape off some of the green powder from a piece of moist bark and 

 mount in water. Draw a single plant under high power, showing the thick 

 cellulose wall and the chloroplasts. 



"2. Notice that the cells (individuals) have a tendency to hang together for 

 some time after division. Study and draw aggregates or colonies of two, three, 

 four, and eight cells still united. In how many directions do the cells divide ? 

 Describe the color, shape, and habitat of the plant. How does it get its food .'' 

 Notice that it must be expose^ to long periods of drouth. 



a b <= O 



Fig. 2.— Life Cycle of Pleurococcus. 



.3. Its life cycle may be stated as follows : increase in size of the cell, division 

 of the cell into two, separation of the daughter cells. Taking no account of the 

 fact that the cells hang together for some time after division, make a diagram 

 illustrating this as indicated in Fig. 2, a. 



4. Make a diagram showing the ancestors of one individual for ten genera- 

 tions. See Fig. 2, b. 



5. Make a diagram showing the descendants of one individual for ten 

 generations. See Fig. 2, c. 



6. All plants and animals, whether high or low, are single cells in the first 

 stage of their life. Therefore, in the higher forms, the egg or spore also passes 

 through the two, four, etc., celled stages, and in these first stages the cells may also 

 represent a loose aggregate or colony, since in many cases, if the cells are 

 separated from each other by artificial means, two or more plants or animals may 

 be obtained from the egg, which would otherwise have produced only one 

 individual. 



Pleurococcus shows how it is possible for a plant to pass from a unicellular 



