66 PLANT GEOGRAPHY 



have very often a spiral twist, and that the seed is 

 commonly excentric, so that the whole structure rotates 

 like a screw-propeller as it falls from the tree, and is the 

 more readily carried horizontally i.e. away from the 

 shade of its parent by the slightest breeze. It may 

 also be noted that these anemochoric structures occur 

 in a great number of quite unrelated Natural Orders. 

 This is true also of each class of dispersive adapta- 

 tions. 



That most normal dispersive adaptations are merely 

 fitted to carry plants a small distance at a time is 

 exemplified by the Thistles, in which the relatively heavy, 

 well-filled grain a one-seeded fruit readily detaches 

 itself from the ring of pappus-hairs, thus commonly 

 dropping to earth but a few yards from the parent plant. 

 At the same time the action of ocean-currents, floating 

 ice carrying earth, tornadoes, or strong persistent winds, 

 must not be ignored. 



ZOOCHORES. Adaptations to dispersal by animal 

 agency fall mostly into two classes: burs, or hooked 

 structures, which become entangled in the hair of passing 

 animals; and succulence, often accompanied by bright 

 colour, which attracts birds. No burs occur on aquatic 

 plants, or on those over four feet in height, these being 

 obviously out of the way of hairy animals. The hooks 

 which originate in a loop in the style of our common 

 roadside species Geum urbanum L., and the twelve or 

 more ferocious harpoons with recurved hooks on the 

 Grapple-plant of South Africa (Harpagophytum procum- 

 bens DC.), which is said sometimes to prove fatal to 

 the lion, are only two among many varied and complex 

 mechanisms. 



Seed-eating birds have muscular gizzards, so that most 

 seeds swallowed by them are destroyed; but fruit-eating 

 species swallow seeds whole, and the testa serves to 

 protect them from the action of the gastric juices. To 

 a less extent the similar swallowing of seeds, which are 

 afterwards ejected uninjured, is carried out by other 

 animals, as, for example, the eating of crab-apples by 

 deer, and the introduction of grasses into new areas by 

 locusts. Even more curious are the established cases 

 of animal-dispersal by mistake, as when ants carry the 

 seeds of the Cow- wheat (Melampyrum) to the fine tilth 

 of the ant-hill, mistaking them for cocoons, and when 



