GREEN ALG^E 233 



These threads are delicate and silky in appearance and some- 

 what slimy to the touch when lifted by the hand. 



363. The spirogyra cell.* The thread of spirogyra is com- 

 posed of similar cells, and a study of one cell is sufficient to show 

 the structure of all in the vegetative or growing stage. The most 

 prominent part of the cell is the chlorophyll body, which is in the 

 form of a spiral band and lies next the inside of the cell wall 

 coiled around in the cell. The band is more or less irregular 

 along the edges. The number of these chlorophyll bands in a 

 cell varies in the different species from one to five or six, while in 

 a given species the number may vary from one to two, in another 

 from two to three, and so on. At quite regular intervals in the 

 chlorophyll band there is a round shining nodule-like body called 

 a pyrenoid. The pyrenoid is peculiar to the algae, being found in 

 many of them. Outside of the algae it is found only in a few 

 species of liverworts. The pyrenoids are probably reserve food 

 products of a protein nature. The pyrenoid in spirogyra is often 

 surrounded by a layer of fine starch grains. Near the center of 

 the cell is a large body of a more or less granular nature and color- 

 less. This is the nucleus of the cell. It is elliptical to convex in 

 form or in outline as seen in profile in some species, and angular 

 in others. Within each nucleus is a nucleolus (sometimes two or 

 three nucleoli). In the living cell these usually appear more 

 dense than the substance of the nucleus and highly refringent. 

 The protoplasm of the cell is a viscid granular substance forming 

 a thin layer next the wall which is connected by strands of the 

 same substance which radiate out from a granular layer surround- 

 ing the nucleus. These strands can be distinctly seen, as they 

 radiate out from the angles of the nucleus, and divide into more 

 slender strands which terminate in the wall layer of protoplasm 

 at a point where the pyrenoid is located in the band. The spaces 

 between the strands, which here are quite large, are filled with a 

 watery fluid, the cell sap. The term protoplasm was earlier used 

 to include the nucleus, which was supposed to be only a denser 



* This paragraph is chiefly for reference, but advanced classes may be 

 able to include it. 



