20 THE BIOLOGY OF FLOWERING PLANTS 



bacterial flora, e.g. moisture, temperature, aeration, reaction, 

 amount of organic compounds. 



Most soil bacteria are destructive, in the sense that they 

 carry on various stages of the disintegration of complex 

 organic bodies, such as proteins and carbohydrates. The 

 end product of one bacterial species serves as the raw 

 material for another, and in the end a carbohydrate may be 

 entirely resolved into carbon dioxide and water, or a protein 

 into carbon dioxide, nitrogen, water, and inorganic 

 compounds of sulphur and phosphorus. Such reactions, 

 at all stages, are continually in process, and may represent 

 the activities of the majority of the soil bacteria. One 

 group deserves special mention as being most intimately 

 related to fertility— the bacteria which deal with nitrogen 

 compounds. Proteins are broken down to amino-acids, 

 and these, and urea, to ammonia {ammontficatwn) by 

 various putrifying forms. The ammonia is oxidised to 

 nitrite, and the nitrite to nitrate, by two autotrophic (assimi- 

 lating carbon dioxide) bacteria, Nitrosomonas and Nitro- 

 bacter (nitrification), the most favourable source of nitrogen 

 for higher plants being thus formed. Acting in the opposite 

 and, to higher plants, unfavourable, direction are bacteria 

 which reduce nitrates to gaseous nitrogen {de?iitrification). 

 The denitrifying bacteria are most active in badly aerated 

 soils. Finally, several soil bacteria [Azotohacter chroococ- 

 cum and Clostridium Pasteuriantim) assimilate atmospheric 

 nitrogen, converting it into organic compounds which 

 ultimately add to the combined nitrogen of the soil (nitrogen 

 fixation). The extent of nitrogen fixation or of nitrification 

 in a soil is a good measure of the bacterial activity favourable 

 to higher plants and, indeed, to the fertility of the soil. 



Soil Temperature. — The temperature of the soil has 

 marked characteristics which change with constitution, degree 

 of moisture, and the nature of the vegetable covering. 

 Russell sums up as follows : " The temperature curve of 

 the soil at a depth of 6 inches below the surface somewhat 

 resembles that of the air in summer, but it lacks the sharp 

 peaks and depressions. The soil minimum is always 



