19S0 Journal of Applied Microscopy 



The plant continues to supply the bacteria with carbohydrates, which it can build 

 up from CO 2 and HjO, and in return receives the surplus nitrogen of the 

 bacteria. 



Leguminous plants, when they die, always return more fixed nitrogen to the 

 soil than they take away from it, and in this way the cycle of life is perfected." 



The plants are then shown to the class and cultures of nitrogen fixers are 

 provided for examination. B. H. Buxton. 



Cornell University, Medical College. 



A Method for Fixing and Sectioning Bacterial Colonies, 

 Fungus Mycelium, Etc. 



In determining the manner of growth of some bacteria, I have occasionally 

 found it desirable to section an entire colony. This can be accomplished by the 

 following method, which is equally well applicable to the sectioning of fungus 

 mycelium : 



Grow the bacteria or fungus in nutrient agar in a Petri dish. One percent, 

 agar is sufficiently dense, and enough should be poured into the dish to give a 

 thickness of about '1 to 4 mm. When growth has proceeded to the desired stage, 

 cut out blocks of the agar from 3 to (i mm. square, and drop them into fixing 

 fluid. I have used Flemming's, Hermann's, and the mixture described below, 

 with good results. Wash, dehydrate, and embed as for any tissue, keeping in 

 mind that the agar block is rather difficult to penetrate, and the time for dehy- 

 drating and infiltrating must be increased to two or three times its usual length. 



Where it is necessary to " set " a bacterial colony to prevent the washing off 

 or diffusion of the bacteria, a fixing fluid composed of 5() c. c. absolute alcohol, 

 30 c. c. chloroform, and 15 c. c. acetic acid, I have found to be very satisfactory, 

 even where very delicate fixation was desired. I^ix tissues four to twelve hours. 

 Tissues must then be washed in alcohol, and it is best to make the transition 

 from fixing fluid to absolute alcohol gradually. That is, pour out about one-third 

 of the fixing fluid and add about an equal quantity of 95 per cent, alcohol ; allow 

 the tissues to stand a few hours in this mixture, pour out more and add more 

 alcohol, and finally transfer to absolute alcohol. Karl Kellerman. 



Bureau of Plant Industry, Washington, D. C. 



