FIXATION OF NITROGEN BY BACTERIA 203 



When grown on artificial media the nodule organisms are at 

 first minute rods, single or in pairs, actively motile, each posses- 

 sing a single polar flagellum, hence their inclusion in the genus 

 Pseudomonas. Later they lose their motile power and change 

 into bacteroids, becoming irregular in form, curved, and some- 

 times more or less branched. They often excrete a mucilaginous 

 substance ; the protoplasm is of unequal density and does not 

 stain uniformly. 



Wood-Ash-Maltose solution. In this solution the organisms 

 flourish, the medium becoming turbid and ropy with a whitish 

 growth in it which accumulates on the bottom of the flask. 



Agar. On Wood-ash-maltose agar (Ex. 99) plates the surface 

 colonies are round, raised, with a translucent appearance very 

 like drops of melted paraffin wax. They may grow to a diameter 

 of 2 or 3 mm. in five to ten days, and at first are watery, but 

 after a time develop a slimy substance which can be drawn out 

 into long threads. The deep colonies are smaller, round, oval 

 or triangular, granular and whitish. 



Ex. 94. Dig up well-grown specimens of beans, red clover, 

 alsike, kidney - vetch, birds' -foot trefoil, sanfoin, and other 

 leguminous plants just before they flower : wash the roots care- 

 fully, and make observations upon the number, position, size, 

 form and colour of the nodules present. 



Ex. 95. Examine the nodules on the roots of beans and 

 peas (i) when the plants are a month old; (2) when they are 

 just coming into flower ; and (3) when the seeds are almost ripe. 

 Cut sections of the nodules, and note the difference in the ex- 

 ternal and internal colour and texture of each at the different 

 stages of growth of the plants. 



Ex. 96. Cut transverse and longitudinal sections of a bean 

 nodule. Examine with a low power, and make drawings indi- 

 cating the position and form of the various tissues. Then 

 examine with a high power, and make detailed drawings of the 

 several parts. 



