I00 GENERAL BIOLOGY 



directly from the air. They supply nitrogen to the clover, 

 and thus repay the debt imposed by the parasitic life. 

 They enable the host plants to grow upon soils that are poor 

 in nitrogen, and by their decomposition they leave such 

 soils richer than they found them. 



Within the galls, or tubercles, these bacteria grow 

 larger than other forms, the cells becoming irregularly rod- 

 shaped, x-shaped, y-shaped, etc. Hence they are easily 

 recognizable with the microscope. Upon examination of 

 the large tubercle) we ordinarily find them filling the in- 

 terior. Upon the dissolution of their bodies, their 

 nitrogen content is added to the soil, either directly, 

 during the growing season, or indirectly through the 

 intermediary agency of the clover. 



Study Jj. A few observations on bacteria. 



Materials needed: A hay infusion a few days old; some 

 growing clover, or other leguminous plant .bearing root tuber- 

 cles: a stock of sterilized culture dishes ready for sowing. 



Mount a little bit of bacterial jelly from the surface of the 

 hay infusion, and survey it for bacteria of the three form- 

 types. Look also for species of any type that may be dis- 

 tingushable by size, cell proportions, etc. 



Clean some root tubercles, split open ; mount scrapings 

 from their interior and study the bacteria in them . 



Make a few cultures on plates of agar as follows : 



1. Seal one sterilized plate without opening, for a check. 



2. Touch all your fingertips to the surface of the agar 

 in a second plate, cover again, and set aside to incubate. 



3. Wash the hands carefully and wipe dry on a clean 

 towel, and repeat. 



4. Capture a live fly, preferably from a dusty window; 

 put it inside a culture plate and let it walk about a little, 

 over the surface of the gelatine to distribute bacteria from 

 its feet; remove the fly and set the plate aside to incubate. 



