198 BRITISH PLANTS 



6. Distribution of Seeds by Man. — Man, intentionally or 

 othftrwise, is not the least important of seed-distributors. 

 Wherever his merchandize goes, the seeds of his native 

 flora accompany it. For this reason a rich aUen flora 

 is generally developed in waste places about docks. 

 Most of these strangers soon die out, but here and there 

 some flourish and establish a permanent foothold in the 

 country — e.g., the thistle in Australia. 



Of aU the agents of seed-dispersal, the most efficient, 

 excluding man, are the birds and the wind. If the seeds 

 are heavy, the wind carries or drives them a very short 

 distance ; explosive fruits seldom eject the seed beyond a 

 few feet ; creeping fruits cannot travel far ; animals do 

 not generally wander far from their homes, physical 

 barriers, natural enernies, and family ties seriously limiting 

 their freedom. But the paths of the air are free, and 

 oppose no obstacles to flight. Birds travel through the 

 air much faster and much farther than animals traverse 

 the land. So efficient is this method of dispersal, 

 especially for large and heavy seeds, that plants whose 

 seeds are so dispersed can afford to expend a large part 

 of their substance in making large fleshy envelopes for 

 a small number of seeds, instead of using up all the food 

 in the production of as many seeds as possible. Migratory 

 birds travel every year enormous distances, but as the 

 migrations are north and south, the birds pass through 

 latitudes which differ so widely in chmate and seasons 

 that the seeds they may bear with them are seldom 

 capable of establishing a successful footing where they 

 fall. Small light seeds, however, especially if equipped 

 with floats, are carried a long way by the wind. These 

 also have the advantage of being produced in great 

 numbers. This accounts for, the fact that plants possess- 

 ing small wind-borne seeds, as a rule, travel faster along 

 the roads of conquest than those which depend for their 

 dispersion on birds. Moreover,, the wind follows the 

 lines of latitude more closely than birds, with the result 

 that the dispersal of wind-borne seeds is effected through 

 more uniform climates than seeds carried by birds in 

 their migrations. 



