A MANUAL OF BOTANY 99 



(filament) of the plant? Try to follow a single fila- 

 ment from end to end. Look very carefully for markings 

 of any kind along the filament. You ought to be able to 

 distinguish two kinds of these, viz. : (1) cross lines at 

 equal distances, really circular partitions between the cells, 

 and (2) what appear to be zigzag lines. What are the 

 colors of the two kinds of lines ? What color has the re- 

 maining ])art of each cell ? 



Demonstration by a student : Let a student take a large 

 open-moutlied cylindrical jar and with a piece of green chalk 

 draw a spiral line on the inner surface. Compare the 

 appearance of this mark with the ai)pearance of the zigzag 

 line of the cell, and from it deduce the true nature and 

 position of the marking. 



Effect of iodine. Apply diluted iodine to your prepara- 

 tion and observe the change in appearance. Are the 

 spiral markings more or less distinct than before ? What 

 color do they now have ? What do you observe scattered 

 along their course? These spiral bands are chloroplasts 

 (green substance) and the round or oval bodies are nodules. 



Nodules. Examine a nodule (Jqi) in an unstained speci- 

 men. Examine the same specimen stained with iodine. 

 You observe a change of color, really to a bluish purple, 

 if the spirogyra has been in the sunlight. 



Experiment. Take a little starch, mix with water, and 

 add a drup of dilute iodine. What do you observe and 

 what do you now infer concerning nodules ? The experi- 

 ment is the test for starch. Eemember it. 



Cell structure (hj)). With an iodine stained specimen and 

 (hp) examine the cell very carefully for a brownish body 

 near the center, the nucleus. Observe if possible its exact 

 shape, the strands attaching it to the spiral band, and a 

 round spot, the nucleolus, within it. Observe any irregularity 

 of the chloroplast and how it terminates at each end of the 

 cell. Does it continue into the next cell or does it stop ? 



