4(32 JOUitXAL OF l-'OKl'.STKN' 



that in the acceptance of the above definition the word habitat should 

 be defined as precisely as possible. The author's definition of habitat 

 is "any unit area in which the combined influence of the various exter- 

 nal factors which determine the ecological aspect of the vegetation is 

 such as to produce an essentially uniform environment." He further 

 defines the habitat as any unit area in which the combined influence of 

 the climatic, edaphic, and biotic factors is essentially uniform through- 

 out. With this precise interpretation, he states that a salt marsh, a pond 

 or a ravine, which frequently is characterized as a habitat, should be re- 

 corded rather as a series of habitats. By applying the law of the mini- 

 mum to problems in local physiographic ecology, climatic factors need 

 not be taken into account, since thc)^ are essentially constant through- 

 out the region, the variable factors being chiefly edaphic, due to varia- 

 tions in soil and topography. In problems of regional physiographic 

 ecology, however, the climatic factors are variable and must be taken 

 into account. y ^^ f 



The Plant World, Vol. XX, Oct. and Nov., 1917, pp. 305-319 and 341-353. 



SILVICULTURE, PROTECTION, AND EXTENSION 



Pennsylvania for many years has been far in 

 Planting advance of all other States in the extent to which 



Scotch Pine silviculture is applied in the care and manage- 

 in ment of State forests. Professor Illick, in the 



Pennsylvania article under review, not only discusses the use 

 of Scotch pine in Pennsylvania forestry, but 

 shows in a series of tables the extent to which the planting of trees on 

 the State forests has been done each year since 1899. In 1916, 

 5,492,020 trees were planted on the State forests. Of the 22,045,311 

 planted to the end of the year 1916. approximately two-thirds were 

 white pine. Next to white pine in the total number planted were Nor- 

 way spruce and Scotch pine. The tabulation below gives the annual 

 planting in Scotch pine in Pennsylvania since 1909: 



Number of 

 Year trees planted 



1909 74,404 



I9IO 70,925 



I9II 98,576 



I912 152,900 



1913 91,050 



I914 133,154 



I915 18,775 



1916 714,950 



Total 1,354,734 I 



