PLANT CLASSIFICATION 



167 



microscope each of these threads is seen to be a series of cells joined 

 end to end. All the cells are practically the same in shape and 

 structure, however, so that a study of one will make clear the struc- 

 ture of all. 



Inclosing each cell there is a thin cell wall. The first structures 

 one is likely to notice within the cell are the chlorophyll bodies. 

 In the pond scum known as Spirogyra the chlorophyll is arranged 

 in spiral bands, and it is this which has given the plant its name 

 (Fig. 85, B). In other fbrms the chlorophyll is differently arranged, 



chlorophyll band 



nucleus 



' chlorophyll band 



FIG. 85. Spirogyra. (Strasburger.) 

 A, two conjugating threads of Spirogyra ; B, single cell of Spirogyra. 



sometimes in star-shaped masses, one in each half of the cell, and 

 sometimes diffused throughout the cell. If a little iodine is added 

 to the specimen when it is being examined under the microscope, 

 a nucleus may be distinguished near the center of each cell (Fig. 85, 

 B}. In the cell-body and nucleus the protoplasm appears as a 

 clear and almost transparent mass. 



