GEEEN ALGJE. 



31 



is shown by extracting the chlorophyll, which may be done by placing the 

 plants in alcohol for a short time. This extracts the chlorophyll, but a 

 microscopic examination of the decolored cells shows that the bands 

 remain unchanged, except for the absence of color. These bands are 

 flattened, with irregularly scalloped margins, and at intervals have 

 rounded bodies (pyrenoids) imbedded in them (Fig. 18, 0, py.). The 

 pyrenoids, especially when the plant has been exposed to the light for 

 some time, are surrounded by a circle of small granules, which become 



FIG. 18. A , a filament of a common pond scum (Spirogyra) separating into 

 two parts. B, a cell undergoing division. The cell is seen in optical section, 

 and the chlorophyll bands are omitted, n, n', the two nuclei. C, a complete 

 cell, n, nucleus, py. pyrenoid. D, E, successive stages in the process of 

 conjugation. G, a ripe spore. H, a form in which conjugation takes place 

 between the cells of the same filament. All x 150. 



bluish when iodine is applied, showing them to be starch. (To show the 

 effect of iodine on starch on a large scale, mix a little flour, which is nearly 

 all starch, with water, and add a little iodine. The starch will immediately 

 become colored blue, varying in intensity with the amount of iodine.) The 

 cells divide much as in Cladophora, but the nucleus here takes part in the 

 process. The division naturally occurs only at night, but by reducing the 

 temperature at night to near the freezing point (4 C., or a little lower), the 

 process may be checked. The experiment is most conveniently made when 



