REVIEWS AND NOTICES OF BOOKS. 239 



Beyerenick, and others on soil biology. Speaking of the gain of 

 nitrogen in forest soils, due consideration is given to, among 

 other agencies, the part which the recently discovered organism 

 Azotobacter chroococcum plays in this process. 



Prominence is also given to the absence of nitrifying bacteria, 

 through conditions inimical to their growth, and to the presence 

 of ammonia-forming bacteria, which are abundantly present in 

 forest soils. One is left, however, with an impression that much 

 more investigation into the sources of assimilable nitrogen by 

 forest trees is needed. Useful tables on the composition of 

 leaves at different ages, and of different varieties of trees, are 

 given. Chapter xi. deals with the chemical composition of forest 

 soils. The elementary requirements of forest growth in essential 

 mineral constituents are, of course, the same as those of other 

 plants, and they are dealt with here. The section on chalk and 

 nitrogen is most instructive. In connection with the latter, the 

 likely gains and losses under different circumstances are 

 enumerated, and reference is made to the work of Hellriegel, 

 Wilfarth, Grandeau, Lawes and Gilbert, Maz^ Nobbe, Hiltner 

 and others, along with a criticism of Jameson's hypothesis. 



The next forty pages are devoted to the exhaustion of the soil 

 by forest vegetation. Numerous tables are given, and frequent 

 reference is made to the work of Ebermayer, Schroder and Waber 

 on this subject. The exhaustion of the soil under silviculture, with 

 its periodic harvests, is almost insignificant compared with that 

 of soils under agriculture with its annual harvests (see Table H., 

 p. 250). In horticulture, the soil frequently requires recuperation 

 by means of manure. The physical properties of soils, and the 

 importance of water in forest growth, are considered in chapters 

 xiii. and xiv. In forestry, the texture of the soil is relatively far 

 more important than the chemical composition, and this question 

 is adequately dealt with here. Rainfall in connection with forest 

 distribution makes several pages of very instructive reading, and 

 an interesting discussion follows on the amount of water required 

 per gram of dry matter formed. Trees are great consumers of 

 water, and this fact is well illustrated in chapter xv., where, by 

 means of graphs, it is shown that the water-level is permanently 

 lower when supporting than when not supporting forest growth. 



The classification of forest soils, according to their geological 

 origin, and the distribution of the forests in France, along with a 

 detailed study of several typical soils and forests, occupy the 



