138 ADAPTATIONS. OECOLOGICAL CLASSIFICATION SECT, in 



Finnish botanists have adopted a kind of nomenclature to denote -the different 

 strata or storeys : 

 Tree-stratum 

 Bush-stratum . .90 centimetres to about 4-5 metres 



Tallest field-stratum 

 Middle field-stratum 

 Lower field-stratum 

 Ground stratum 



45 to 80-90 centimetres 



10 to 45 



5 to 10 



immediately on the surface of the soil up 



to 5 centimetres (mosses, lichens, algae). 

 It may, however, suffice to consider merely four strata : 



i. Ground-stratum : immediately above the soil : mainly mosses, lichens, and algae. 



ii. Field-stratum : formed by grass and herbs, as Well as dwarf shrubs of 

 approximately the same stature. 



iii. Shrub-stratum : formed of taller shrubs. 



iv. Tree-stratum. 1 



4. Colour of vegetation. We may compare brown heath with 

 green meadow. Here, too, may be mentioned the colours of flowers, 

 and the contrast between entomophily and anemophily. 



5. Seasonal relationships. This involves the duration of the 

 resting period, and other phases of vegetation (foliation, flowering, and 

 defoliation). We may compare evergreen forests with those that shed 

 their foliage for winter, or for the dry season ; steppe which is green 

 for a few months, but yellow-brown and bare for much longer ; the 

 vegetation of north-temperate Europe in winter, and in summer. 



6. Duration of life of species : duration of epigeous parts, the 

 role played by annual and renascent species, and by woody plants. 2 

 Only rarely do we find an assemblage of plants consisting solely of annual 

 species, but they do occur, as in the case of Salicornia herbacea, and 

 of certain weeds extending over limited areas. 



7. The number of species present gives some indication of the 

 struggles for space between species ; this struggle may be greatly inter- 

 fered with, and is, in fact, interrupted by man. Sometimes as in 

 spruce-forest, beech-forest, and ling-heath a single definite species 

 dominates ; at other times there is an extraordinary admixture of species. 

 Rich in species are communities in warm countries, such as tropical 

 forests, 3 the maquis of Cape Colony ; poor in species are the communities 

 of northern Europe. It is evident that favourable conditions of life 

 call into existence a more complex flora ; but geological factors have 

 often played some part. 



In the vicinity of Lagoa Santa in Brazil, on an area of about three geographical 

 square miles, there grow about 3,000 species of Vascular plants (since more than 

 2,600 species have been determined, and there must be at least 400 species that 

 have not been collected). Of these there are 1,600 species (circa) in the forest 

 and 800 species (circa) in the campos, of which 400 and 90 respectively are trees; 

 such is the case, despite of the fact that the area of forest is much smaller than 

 the area of the campos, and is essentially confined to the valleys where it bounds 

 all the watercourses. The reason for the greater richness of this forest-flora is 

 certainly to be sought in the prevailing physical conditions (more abundant humidity 

 and food-material, especially humus). But possibly geological causes have played 

 a part ; for probably the forest-flora is the older, and the flora of the campos 

 gradually arose later, as South America raised itself more and more above the 

 sea, and Brazil consequently acquired a more continental climate. 4 



1 See A. Nilsson, 1902 a. * See p. 8. 



3 See Humboldt, 1 807. See Warming, 1 892, 1 899. 



