112 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



by a resistant covering, in which case germination is often hastened 

 by passage through an animal. For ectozoic dispersal the dis- 

 seminule is commonly adhesive by means of a sticky surface or, 

 more often, by its possession of hooks or other devices by which it 

 catches on to the fur etc. Anyone who has tried to extract the 

 fruits of Burdocks {Arctium spp.) or Beggar-ticks {Bidens spp.) 

 from woolly garments will be aware of the effectiveness of such 

 adhesion. 



Some examples of disseminules modified for dispersal by animals 

 are shown in Fig. 27. 



In addition to such ' official ' types of dispersal there is the frequent 

 ' pecking apart ' by birds : for example, of the seeds or fruitlets 

 contained in Apples and Rose-hips. There is also the transport 

 of materials for nest-building, and the still more fortuitous adhesion 

 of disseminules to the feet, etc., of animals in mud and clay or by 

 freezing to their fur or feathers. Thus, for example, Darwin {op. 

 cit., p. 328) mentions removing a considerable amount of clayey 

 earth from the feet of Partridges, reporting that in one instance 

 around the wounded leg and foot there was ' a ball of hard earth 

 adhering . . . weighing six and a half ounces . . . but when . . . 

 broken, watered and placed under a bell glass, no less than 82 plants 

 sprung from it. . . . With such facts before us, can we doubt that 

 the many birds which are annually blown by gales across great 

 spaces of ocean, and which annually migrate — for instance, the 

 millions of quails across the Mediterranean — must occasionally 

 transport a few seeds embedded in dirt adhering to their feet or 

 beaks ? ' Quite apart from this, Rabbits, etc., will often drag twigs 

 for some distance when these are attached to their fur, and Water- 

 fowl have frequently been observed carrying sizeable pieces of 

 Pondweeds {Potamogeton spp.) on their backs or around their necks 

 — even when in flight. 



{a) Birds. On account of their abundance almost everywhere in 

 the world, of the very great distances which many regularly fly, 

 and of their consequent power to cross wide expanses of water. 

 Birds tend to be the most important group of animals from the point 

 of view of plant dispersal. Although it has been contended by some 

 authors that Birds ' fly clean ' on migration, this does not seem to be 

 always the case ; indeed, according to Ridley {op. cit. p. 444), it is 

 ' strongly negatived by much evidence '. Moreover, they are apt to 

 ' neglect their toilet ' when unwell, and similarly can have materials 

 sticking or frozen to their beaks, feet, or feathers when flushed or 



