66 THE STORY OF PLANT LIFE 



The present-day distribution of plants is, of course, 

 also dependent upon the constitution of the floras of 

 the past. These have undergone change owing to 

 the alteration in the distribution of land and water, 

 and the alterations of climate. The following table 

 gives a summary of the character of the conditions 

 of surface whether land or sea in past ages, and the 

 nature of the climate so far as we can approximately 

 consider it to have been constant at any one period. 



The table on page 69 shows the probable area of the 

 different geological formations and their representa- 

 tion in one or more of the main zones of vegetation 

 according to climate. The vegetation of lofty hills 

 in the tropics is a repetition of the vegetation of the 

 lowlands as we travel from the equator to the poles, 

 showing that altitude as well as climate is of 

 importance in determining the distribution of 

 plants. 



19. Plant Formations. 



The study of systematic botany is concerned with 

 the study of the species, which in floristic plant 

 geography therefore constitutes the unit. Plant 

 geography, however, also embraces ecology, or the 

 study of the habitats of plants, the communities 

 formed by plants, and the various other problems 

 relating to the connection between plants and their 

 environment, studies which are physiological in part 

 as well as morphological. 



The unit in ecology is then the particular type of 



