1976 



Journal of Applied Microscopy 



We select them for our class : 1. A control flask. '2. One to show oxidation 

 of ammonia to nitrites. 3. One to show oxidation of nitrites to nitrates. 



By way of preliminaries to the demonstration of fixation of nitrogen some 

 alfalfa plants are started growing 2 or 3 months beforehand, and the proceedings 

 are as follows : 



River sand is thoroughly washed, dried, and sterilized at 150°C. for two 

 hours. Then put into garden pots (we used tin cases in which photographic 

 platinotype paper had been stored), and boiled for an hour or so in the steam 

 sterilizer. The alfalfa seeds are sterilized in 5 percent, formalin for 15 minutes, 

 washed in two or three changes of sterilized water, sown in the sand, and kept 

 watered with sterile mineral solutions, containing no nitrogen, the one used by 

 preference being : 



Tricalcic phosphate, - - - - .5 gram. 



Calcium sulfate, .5 gram. 



Sodium chloride, .5 gram. 



Potassium sulfate, - - - - .5 gram. 



Magnesium sulfate, - - - - .5 gram. 



Iron sulfate, ------ Trace. 



Water, - lOOO c c. 



One-half of the seeded pots are supplied with pure cultures of nitrogen fixers, 



A L F A L ^ A 



SIM k'^irV Ml DItAI. COI.I.l-.OF. 



Fig. 1. — Left. — Alfalfa Grown in Sterile Sand. Right. — Same with Cultures 

 of Nitrogen Fixers. 



isolated from alfalfa rootlets, two or three times during the first two or three 

 weeks of growth, the remaining pots not being treated. 



No precautions need be taken to prevent air bacteria from falling in during 

 the period of growth, but it is advisable to keep the untreated pots in a separate 

 room where no cultures of nitrogen fixers are handled. Conditions, however, of 

 lighting and temperature should be the same for both. 



The photographs (Figs. 1 and 2) show the appearance three months after 

 sowing. It is true the growth is not very luxuriant in the treated pot, but a 



