SPIROGYRA 339 



with a clearly defined cell wall. The contents <>f the cell 

 are green, due to the chlorophyll which conceals all parts «»t' 

 the cell except the nucleus. Tin; nucleus usually li«-^ uear 

 the center of the cell. As lun«r as the cell is full of chlo- 

 rophyll, the cytoplasm cannot be Been ( Figure 363). 



Pleurococcus makes its own food as tic bean does, ami 

 apparently it is able to digest the starch and protein which 

 it makes in a manner similar to that of the bean. When- 

 ever a number of pleurococcus cells are examined, some 

 are found to be dividing. In this division the nucleus 

 forms two nuclei which move apart. A partition wall 

 forms and two cells take the place of the old or parent 

 cell. This method is called fission (Latin, Jixsn*\ cleft ), 

 and is the simplest form of reproduction. In pleurococcus 

 the cells do not always separate at once, but form groups 

 of two, three, or four cells (Figure 363). 



SUMMARY 



This simple unicellular (one-celled) green plant, pleu- 

 rococcus, lives and makes its own food and grows new 

 cells. While there are no flowers and seeds as in tie- 

 bean, yet this plant is able to reproduce itself. All of 

 the important life processes found in the bean take place 

 in the simple, single cell. 



LABORATORY STUDY OF PLEUROCOCCUS 



Study this as an example of a plant which consists of a Bingle cell, 

 but still performs all the processes common t.> higher plants. Soak :i bit 

 of bark and scrape it ucntly to ^et the pleurococci cells, some ot which 

 may be in groups. Draw a single cell ami a group of .ells. 



240. Spirogyra. — This plant is best known as the "pond 



scum " which l;to\vs in most fresh water ponds and in 

 slow running streams. It may be kepi for some time in 

 glass dishes in a laboratory. Instead of being made up of 



