20 



CHAPTER III. 



Means of Dispersal. 



WHEN the adaptation of plants for dispersal is spoken 

 of, one thinks of winged seeds, or of clinging burrs, of 

 floating nuts, of succulent fruits which tempt birds, or of 

 other obvious adaptations. These, however, form only a 

 few of the contrivances made use of by nature to aid 

 plants to hold their own and to extend their range. On 

 considering what is necessary to the existence of a species, 

 it soon becomes evident that modes of dispersal that seem 

 to be merely accidental really depend on some modifi- 

 cation of the seed or plant. They are often alternative 

 methods without which the very life of the species would 

 be in danger. 



No plant of the Temperate Regions — I do not speak 

 of Tropical species — would be likely to hold its own for 

 long periods if it were confined to a single station. The 

 sweeping climatic waves which time and again have 

 passed over our latitudes within the life-time of the exist- 

 ing species must have compelled every one now found in 

 Britain to move. When deep snow and ice smothered our 

 uplands, the Alpine flora had to descend to the lowlands ; 

 when a warmer climate returned, the Arctic plants had to 

 leave the low ground and again climb the heights The 

 lowland plants, on the other hand, with few exceptions, 

 had to leave the country when the Reindeer, Arctic Fox, 



