14 INTRODUCTION chap, iv 



CHAPTER IV. PLAN OF THIS BOOK 



Whereas geography proper has to give information in regard to the 

 species and the distribution of associations of plants in various parts of 

 the earth, oecological plant-geography treats of the following : 



1. The external factors affecting the plant's economy ; the effects 

 of these factors upon the external and internal structure of the plant, 

 upon the duration of life and other biological relations, and upon the 

 topographical distribution of species. These factors and their effects 

 are discussed in Section I. 



2. The grouping and diagnosis of the plant-communities occurring 

 on the Earth. In connexion with each class the endeavour must be 

 made to discover the determinant factors, the modes in which they are 

 combined, and in which they possibly may replace one another. In 

 Sections II and III the communities are treated generally, and their special 

 treatment follows in Sections IV to XVI. 



3. The struggle between plant-communities. This is dealt with in 

 Section XVII. 



The various factors require to be dealt with separately, although 

 this is a disadvantage, partly because they never work singly but work 

 in complex combination, and partly because it is by no means clear in 

 all cases what must be ascribed to one factor and what to another. Fol- 

 lowing Schouw, one may divide the factors into directly and indirectly 

 operating factors. 



Among the direct factors are 



{a) Geographical factors. It is thus that Drude describes the factors 

 that work over great areas, because they are dependent upon the Earth's 

 course round the sun, and upon the latitude : composition of the air, 

 light, temperature, atmospheric precipitations and humidity, movements 

 of the air. 



(6) Topographical factors Those that operate within smaller, more 

 localized, limits : the chemical and physical nature of the soil the 

 ' edaphic factors ' as Schimper ^ termed them. 



In the first section of this work the following arrangement is 

 adopted : 



Atmospheric factors are treated in Chapters V-VIII, as follows : 



Chapter V. Light. 



,, VI. Temperature. 



,, VII. Atmospheric humidity and precipitations, 



VIII. Movements of the air. 



To the atmospheric factors likewise also belongs the composition of 

 the atmosphere. Excepting as regards the variable humidity of the air, 

 this is very constant over the whole world. The two gases playing the 

 greatest part in plant-life, oxygen and carbon dioxide, are present nearly 

 everywhere in the same relative proportion. The more recent investiga- 

 tions have proved that the relative amount of carbon dioxide at great alti- 

 tudes and on lowlands is the same. And between the amount of carbon 



' Schimper, 1898. 



