76 PLANT GEOGRAPHY 



leaved Coniferce, growing in pure forest, its catkin- 

 bearing, deciduous Amentifercs, and a great variety of 

 herbaceous types. The Tropical is characterised by 

 dense, mixed forests, mostly evergreen, with arborescent 

 Polypetalcs, especially Leguminosce, Meliacecs, and Ano- 

 nacecs, and gigantic Monocotyledons, including Palms, 

 Pandanacece, Scitaminece, and Bamboos, with numerous 

 lianes and epiphytes. The Southern is now broken up 

 into the widely distant floras of extra-tropical South 

 America, South Africa, Australia, and New Zealand, 

 which exhibit some original connection in the presence 

 of RestiacecB and Proteacea. 



SOIL. Differences of soil explain many of the dis- 

 continuities of the occurrence of species within their 

 areas of distribution, such as the absence of the calci- 

 phobe foxglove from Switzerland. In the important 

 particular of water-content, habitats are very liable to 

 undergo changes in course of time, as when an ironstone 

 " pan " forms below a sand and obstructs its natural 

 drainage, or when watercourses, partially dammed by 

 a growth of Sphagnum, saturate or inundate the adjacent 

 area. Soil-barriers are, therefore, not always permanent. 

 A temporary drought may permit xerophytes to spread 

 across ground naturally too wet for them, or a tem- 

 porary inundation may entirely destroy certain species, 

 so far as that locality is concerned. 



SEA. Even narrow straits may prove an insur- 

 mountable barrier in the migration of large-seeded 

 plants, which would usually travel by successive short 

 steps. Though many seeds will withstand the action of 

 sea-water, some will not. Wind and birds may also act 

 constantly in the wrong direction, or be inadequate to 

 carry particular species. 



DISTANCE. Here, however, distance, which must be 

 considered as at least a temporary barrier, shows its 

 effect. Whereas a chain of islands at no very great 

 distance apart will act as temporary landing-places for 

 migratory birds and for wind- or current-borne seeds, a 

 wide stretch of uninterrupted ocean will act as a main 

 obstacle to migration. For example, it has been 

 estimated that from 75 to 90 per cent, of the species 

 which " invade " new ground come from the immediately 

 contiguous areas. Though, too (as we shall see more in 

 detail in the next chapter), the floras of islands, even 



