6lO LABORATORY EXERCISES 



of Cellulose by Bacteria and Fungi. Bull. 266, Bureau of Plant Industry, 1913: 28). 

 — Pare, steam and mash a quantity of potatoes. To 100 grams of mashed potato 

 add 800 c.c. of tap water and steam for one-half hour; filter through cotton. 



Potato solution, c.c 5°° 



Agar, grams IS 



Nutrient solution, c.c Soo 



Potato Agar (Another formula). — Put dean pared potatoes through a meat 

 grinder. To 1000 grams of the potato pulp add an equal quantity of distilled water. 

 Stir thoroughly and let stand in an ice box for an hour, with occasional stirring. 

 Strain through gauze of medium mesh. Make up to three times the weight of the 

 original pulp with distilled water. Strain for one hour, filter through cotton and 

 paper and make up to 3000 c.c. with distilled water. Add ij^ per cent, of agar 

 flour, steam for one hour, filter through cotton and paper, tube and autoclave for 

 fifteen minutes at iis°C. As this potato agar varies widely in acidity, to reduce this 

 variation a large quantity of potato juice made from a uniform lot of Burbank po- 

 tatoes is used. This is placed in looo-c.c. flasks tightly plugged and kept in a refrig- 

 erator. The juice is then made up in agar tubes as needed. It was found that this 

 agar varied less than i per cent, in acidity, changing from +7 to -|-6 during five 

 months. 



LESSON 16 



Starch Agar. — To 800 c.c. of boiling water add 10 grams of potato starch sus- 

 pended in a little cold water. Concentrate by boiling to 500 c.c. This breaks up 

 the starch grains and it should give a nearly transparent starch solution. 



Starch solution, c.c 500 



Nutrient solution (same as for cellulose agar), c.c 500 



Agar, grams 10 



Cellulose Agar (McBeth and Scales: BuU. 266, Bureau of Plant Industry, 

 p. 27). — ^Prepare a liter of dilute ammonium hydroxide solution by adding 3 parts 

 of water to 10 parts of ammonium hydroxide, sp. gr. 0.90. Add a slight excess of 

 copper carbonate and shake, allow to stand over night and then siphon off the super- 

 natant solution. Add 10 grams of unwashed sheet filter paper and shake occasion- 

 ally until the paper is dissolved. DUute to 10 liters and add slowly a i to s solution 

 of HCl, with vigorous shaking until the precipitation of the cellulose is complete. 

 Dilute to 20 liters, allow the cellulose to settle and decant the supernatant liquid. 

 Wash by repeated changes of water, adding HCl each time until the copper color 

 disappears; then wash with water alone until the solution is free from chlorine. 

 Allow it to settle several days and decantoff as much of the clear solution as possible. 

 If the percentage of cellulose is still too low, a portion of the solution is centri- 

 fugalized to bring the cellulose content up to i per cent. 



