THE STUDY OF MICROORGANISMS IN THE SOIL 113 



Remove paraffin from the sections in xylol 1 minute; rinse in 

 95 per cent alcohol 1 minute. 



Immerse in iron alum mordant (2 per cent aqueous solution) 

 4 to 24 hours. Over night is convenient. Rinse in water. 



Stain in haematoxylin^ 1 to 

 6 hours. 



Destain in the mordant until 

 desired differentiation is 

 reached; observe under 

 microscope. 



Wash in running water 20 

 minutes to remove all iron alum 

 and thus prevent fading. 



Counterstain in Licht grim 

 (saturated solution in 95 per 

 cent alcohol). 



Dehydrate in absolute alco- 

 hol, clear in xylol, and mount 

 in balsam. 



Exercise 23 



Effect of Root Nodule Bacteria 

 ON the Growth and the Nitro- 

 gen Content of Alfalfa 



2 -gallon jars 



Prepare eight 

 of clean sand. 



Plant to peas as follows : 



(a) 1, 2, 3, 4, sand uninocul- 

 ated. 



(h) 5, 6, 7, and 8, sand 

 inoculated. 



One week after seeds begin to Fig. 9.— Effect of bacteria on growth 

 1 1 -.nrK L-- ,. of sweet peas. Jar on left not inocu- 



germmate add 100 cubic centl- i^ted. Jar on right inoculated. 



meters per jar of plant food 



minus nitrogen (Crone's Medium 106). This nutrient solution 



should be added at intervals of every 2 weeks or whenever needed. 



1 Heidenhain's Haematoxylin. Dissolve 5 grams of haematoxylin 

 crystals in 400 cubic centimeters of absolute alcohol using heat if necessary. 

 Add 600 cubic centimeters of distilled water and allow the solution to ripen 

 a month or more. 



