2io introduction to plant geography [chap. 



Intraneous, Extraneous, and Other Elements 



It is sometimes useful to classify the forms growing in a particular 

 territory according to whether in each case the occurrence is well 

 within the area of the form [intraneous) or near its periphery 

 [extraneous). For instance, the disjunct arctic species occurring in 

 the White Mountains of New Hampshire are extraneous there but 

 intraneous in their characteristic region of habitation, namely, the 

 Arctic. 



The components of such groupings form special elements, which 

 are severally recognizable in most floras. Thus we may have 

 intraneous and extraneous elements in a flora, a preponderance of 

 either characterizing certain areas. This leads us naturally to con- 

 sideration of specific phytogeographic or floral elements, which are 

 closely related to migration. Ideally each such element is the 

 floristic expression of a territory of limited extent, in that it involves 

 the taxa and phytogeographic groupings characteristic of a given 

 phytogeographic area — such as the Mediterranean region, giving rise 

 to a ' Mediterranean element '. Often, however, it seems preferable 

 to extend this concept of floral elements to include other and much 

 wider applications. 



Before a flora can be divided into its main general elements we 

 must eliminate all aliens and ' wides ' (also called ' polychores ', i.e. 

 species having such an extensive range that they embrace several 

 phytogeographic regions). Then the endemics should be studied 

 and, as far as possible, assigned to their various categories. There- 

 after the remaining species may be divided into groups according 

 to the geographical character of their areas, with the object of 

 determining the regions whence these groups originated, and so 

 perhaps establishing the genesis of the flora. For this grouping, 

 five main principles should be followed and elements sought (apart 

 from those already accounted for) : 



(i) Geographical elements — grouped according to the types of their 

 total areas, their altitudinal ranges, or their distributions within the 

 region concerned. This is, however, often insufficient to determine 

 the origin of a flora that is not a migration one, for relic and endemic 

 elements so grouped do not reflect the genesis of a flora. Even 

 with a marked arctic-alpine element it is often doubtful which way 

 the components have spread — whether they are arctic types which 

 have migrated southward into the mountainous regions offering 

 somewhat comparable conditions, or vice versa. All that can be 



