PRACTICAL WORK 213 



SPIROGYRA. 



(7) Draw a single cell ' on a large scale under the high power, 

 showing cell wall, cytoplasm, chloroplast with pyrenoids, vacuole, 

 nucleus (with nucleolus) and bridles (if present). 



(8) Compare the pyrenoids of two preserved and decolorised 

 samples, one of which, previous to killing has been well 

 illuminated, the other kept for a day or two in the dark. Now 

 stain each with iodine and observe again. Note that the pyrenoid 

 itself stains brown with iodine (it is a protein crystalloid) as 

 contrasted with the starch formed round it which stains dark 

 blue (almost black). 



(9) Examine the conjugation of Spirogyra, fresh if possible, if 

 not in preserved material, and draw as many stages in the process 

 of conjugation as you can find. 



(10) Plasmolyse vegetative Spirogyra * with 5 per cent, calcium 

 chloride solution (ordinary salt solution injures the cells) and draw 

 stages in plasmolysis. Recover by placing in water. 



1 A species with a single chloroplast or two chloroplasts in each 

 cell is the easiest to draw in the first instance. This should be com- 

 pared with a large species such as S. crassa containing seven or eight 

 chloroplasts, and showing bridles running into the pyrenoids. 



J If time permits. 



