General 



If, then, one were prepared to speculate on the original 

 settlement of the earth by the plant world, one would 

 expect the dry land to have been occupied by, first, 

 Algae, Fungi, and Lichens ; 2. Mosses ; 3. Ferns ; 4. 

 Flowering plants in an open flora such as one sees in 

 a desert, in Tibet or in the Arctic Regions ; 5. A close 

 flora of small shrubs ; and 6. Trees or a wood asso- 

 ciation. 



At present the development of flowering plants has 

 reached such a stage that they grow almost every- 

 where, and are adapted to all sorts of climates and 

 conditions ; and yet we find in the colonisation of 

 these lavas, hints which seem to explain the old original 

 process.* 



Something of the same succession can be traced even 

 now on mountain sides and in the Arctic Regions. 

 For, as we shall see in another place, lichens occur on 

 boulders or bare rock in far Northern latitudes, also on 

 mountain ridges where not even mosses can manage to 

 exist. An open or scattered flora of small half shrubby 

 plants characterises both the highest Alpine and most 

 arctic conditions. Heather moors extend farther up 

 the mountain side and more to the north than trees ; 

 and even amongst trees, coniferae, which are relatively 

 older and less complex in structure than the deciduous 

 trees, are often found as mountain and northern woods, 

 when the lowlands are occupied by oak forest or other 

 deciduous woods. 



So that if one assumes that the plant world even 

 to-day is proceeding with, but has not finished its task of 

 occupying the far north and mountain summits, then we 

 see in every case a higher and more advanced kind 



* Tropical lavas show blue -green Algae, Ferns, Rock plants, Shrubs, Jungle. 

 Temperate lavas show Lichens, Mosses, Open Flora, Shrubs, Trees. Old 

 walls show Lichens, Mosses, scattered Herbaceous plants. 



21 



