Probably no subject in the whole domain of cryptogamic botany has wider bearings on agricultural science than the study of the flora and changes on and in manure and soil. Nitrifying bacteria play a very important part by providing plant life with a most necessary food. They occur in the soil, and two kinds have been described--the one kind converting ammonia into nitrous acid, and the other changing nitrous into nitric acid. We are principally indebted to Winogradsky for our knowledge of these bacteria; he furnishes instances of the bearing of bacteriological work on this department of science, and explains, not only the origin of nitre-beds and deposits, but also the way the ammonia compounds fixed by the soil in the neighbourhood of the root-hairs are nitrified, and so rendered directly available to plant life. The investigations of other observers show that the nitrifying organism is a much more highly-developed and complex form than had been suspected; that it can be grown on various media, and that it exhibits considerable polymorphism--_i.e._, it can be made to branch out and show other characteristics of a true fungus. “I have,” writes Professor Ward, “for some time insisted on the fact that river water contains reduced forms of bacteria--_i.e._, forms so altered by exposure to light, changes of temperature, and the low nutritive value of the water, that it is only after prolonged culture in richer food media that their true nature becomes apparent.” Strutzer and Hartleb show that the morphological form of the nitrifying organism can be profoundly altered by just such variations of the conditions described by Ward, and that it occurs as a branched mycelial form; as bacilli or bacteria; or as cocci of various dimensions, according to the conditions. “These observations, and others made on variations in form (polymorphism) in other fungi and bacteria, open out a vast field for further work, and must lead to advancement in our knowledge of these puzzling organisms; they also help us to explain many inconsistencies in the existing systems of classification of the so-called ‘species’ of bacteria as determined by test-tube culture.”