CHAP. XXXV FORMATIONS 141 



between trees, shrubs, dwari-shrubs, undershrubs, herbs, mosses, and the 

 hke.i 



Upon this basis the following types of formations must be dis- 

 tinguished : 



1. Microphyte-formation or thallophyte-formations, in which 

 the community is composed exclusively, or mainly, of lichens and algae. 

 Here, except in the case of sea-weeds, there can scarcely be any question 

 of more than one stratum (storey) of plants. 



2. Moss-formation. Here algae may form a lower stratum (storey). 



3. Herb-formations, such as meadow, prairie, grass-steppe, and others. 

 Here may occur two or several strata (storeys) ; namely, a humbler vege- 

 tation of thallophytes or mosses, and a taller vegetation of herbs ; the 

 herbs may in turn be ranged into storeys of different heights. 



4. Dwarf -shrub formations and undershrub- formations also 

 include admixture of herbs, which sometimes overtop the dwarf-shrubs, 

 and undershrubs. These longer-lived constituents preponderate, however, 

 and among them there may occur several storeys of vegetation belonging 

 to the types described under i, 2, 3. 



5. Bush-wood or Shrub-wood is composed of taller, hgnified, 

 many-stemmed plants. Compared with the previous ones the community 

 gradually has become richer in growth-forms ; there may occur epiphytes 

 and hanes, and below the highest story, all the forms of vegetation 

 of I, 2, 3, 4 ; for instance, mesophilous herbs growing in the shade. Yet 

 the oecological conditions prevaihng in bush are not the most favourable 

 to plant-life, and the ground-vegetation is often very scanty, because 

 bush-wood may be so dense as to permit the passage of even less light 

 than does forest. 



6. Forest is the tallest type, and exhibits the greatest multiplicity 

 of growth-forms as well as the largest number of storeys : 



High forest, composed of trees hght-demanding and shade trees ; - 

 in tropical forest more than one storey. 



Underwood, composed of shrubs, dwarf-shrubs, and undershrubs. 



Forest-floor vegetation, composed of herbs, mosses, thallophytes, and 

 many saprophytes, and depending upon the light, which is more or less 

 enfeebled by the tree-crowns, upon moisture in the soil and air, upon 

 the humus, and other factors. Beneath such species as beech, spruce, 

 and silver fir, which grow close together and cast a deep shade, there 

 is a very meagre vegetation, but below Hght-demanding trees, in harmony 

 with their need for light, there is a richer vegetation. The flora at the 

 edge of a forest may differ materially from that of the interior, because 

 the conditions of illumination in the former position permit the develop- 

 ment of many species that are excluded from the latter. Gri'\-illius ' 

 found that the tall herbs in thinly wooded and therefore well-lighted 

 Scandinavian forests can be ranged into different types, which differ 

 from one another in the arrangement of the inflorescences, the form 

 and position of the assimilatory organs, innovation, time of flowering, 

 and distribution in the different storeys of the general plant-community 

 all these relationships deserve closer attention and investigation. 



' Sec Kerner, 1891. * Sec p. 18, ' Grcvillius. 1894. 



