22 Origin of the British Flora. 



was only comparatively small area suited to their 

 needs. 



Before studying more minutely the means of dispersal 

 available, it may be well to ask, in this connexion, what 

 are the requirements that are usually essential to the life of 

 the species. In the first place, it is necessary that the 

 seed should be sown beyond the limit of the patch of soil 

 exhausted by the parent plant. For this a very slight 

 mobility is requisite. Secondly, in the case of British 

 plants, some method is ordinarily needed by which 

 they are enabled to cross barriers, such as rivers or 

 straits, or tracts of desert in which the plant cannot 

 flourish. 



I use the term ' desert ' as implying areas unfavourable 

 to any particular species. A desert from the human 

 standpoint is a sandy waste without water, which is 

 unsuitable for the plants and animals useful to man. 

 Such an area may be gay with flowers, and is no desert to 

 the Gorse or Horned Poppy — the desert to them is the 

 luxuriant meadow or forest, which they cannot overpass 

 unless their seeds are carried by some rapid messenger. 

 To a water-plant the dry land is a desert ; to a mountain 

 plant the lowlands are desert ; to the lowland plants the 

 mountain is a desert ; and to go further, to certain plants 

 everything but limestone rock is a desert. Consequently 

 the British Isles consist not only of an Archipelago with 

 numerous islands, but from the points of view of different 

 plants the area forms quite different Archipelagos, of low- 

 lands with scattered mountain tops, of non-calcareous 

 country with isolated limestone, or of dry land with scat- 

 tered lakes. 



In gregarious plants, such as heaths and rushes, the 

 necessity for scattering the seeds beyond the shadow of, 

 and beyond the soil exhausted by the parent species, may 



