192 PKACTICAL BOTAKY 



When magnified, the cells that compose Spirogyra are seen 

 to be very large as compared with those of any alga yet stud- 

 ied. They are joined end to end, thus forming the filamentous 

 plant. Each cell has a cylindrical cell wall which contains 

 one or more peculiar spirally arranged chloroplasts, each of 

 which extends almost or quite the entire length of the cell 

 (Fig. 156). Different species of Spirogyra may have different 

 numbers of chloroplasts in each cell, and this is one of the 

 ways of distinguishing the species from one another. If 

 the chloroplast were uncoiled, it would be like a ribbon with 

 the edges more or less indented. A layer of cytoplasm lies 

 just within the wall, and the cytoplasmic threads run from 

 all sides to the nucleus, which the cytoplasm surrounds. Much 

 of the interior of the cell is occupied by one or more vacuoles. 

 The cytoplasmic layer and the nucleus may be made more 

 conspicuous by mounting the plants in an iodine solution, 

 which pulls the cytoplasm away from the cell wall and also 

 stains the nucleus and the threads which support it. 



178. Spirogyra: nutrition. The supply of water for this plant 

 is secured from the surrounding medium in which are dissolved 

 the carbon dioxide and other inorganic materials from which 

 foods are made. Indeed there is much water within the plant. 

 By carefully drying, it may be demonstrated that sometimes 

 as much as 98 per cent of Spirogyra is water. That photo- 

 synthesis is carried on is often made evident by the oxygen 

 bubbles that arise from the active plants. It is possible to 

 collect the oxygen that is produced by alga) approximately, as 

 shown in Fig. 12. A test tube is placed over the small end 

 of a glass funnel, both being under water in order to exclude 

 all air from them. While under water the large end of the 

 funnel is placed over a mass of algse. The apparatus is then 

 made secure and left in an upright position. As the plant con- 

 tinues its work, oxygen bubbles arise and accumulate in the 

 closed end of the tube, thus forcing out a similar volume of 

 water. The oxygen may be tested by the ordinary tests for 

 this gas. Because of the size and the length of this plant, and 



