412 THE BIOLOGY OF FLOWERING PLANTS 



eats it. Birds are the chief dispersing agents, at least in 

 temperate countries, and some are much more destructive 

 than others. The bird, we may note, is able to pick out 

 not only fleshy fruits, but many inconspicuous hard seeds 

 and fruits. Many are destroyed by the pecking, and still 

 more in the powerful gizzard, e.g. of such birds as the duck. 

 According to Birger (1907) most seeds pass undamaged 

 through the digestive canal of birds like the fieldfare. 

 The efficiency of this type of dispersal is shown by the 

 rapid spread of berry-bearing shrubs in woods and on 

 heaths ; the case of the snowberry, Symphoricarpus race- 

 mosus, which has spread vigorously in this country since its 

 introduction from North America, is a good example. 

 Little, however, is known of the distances to which plants 

 may be spread in this fashion ; probably scattering over 

 relatively small areas is of most importance. 



A special case of considerable interest is the dispersal 

 of the seeds of the mistletoe and of other Loranthaceas. 

 These parasites can grow only if the seed is deposited on the 

 branch of a suitable tree. The missel-thrush is specially 

 fond of mistletoe berries but does not eat the seed ; it rubs 

 this off against the branch on which it perches, and the seed 

 becomes fixed by the viscid internal flesh of the berry. 

 The same method of dispersal has been described for 

 Singalese Loranthacese by Keeble (1895). It is also stated 

 that the mistletoe berries are voided by the missel-thrush 

 on branches and are, in this case too, fixed by the undigested 

 slime, but the other method seems to be the more important. 

 A very large number, especially of woodland plants, are 

 dispersed by ants. The ant picks out seeds which are 

 provided with special oil bodies or elaiosoiftes , which may be 

 of diverse morphological nature. Most conspicuous is the 

 arillar type seen in the little orange elaiosome of the seeds 

 of whin or broom, and of which a magnificent example is 

 given by the African lucky bean, Afzelia (Fig. 62). Ser- 

 nander (1906), to whom we owe a detailed monograph of 

 this mode of dispersal, distinguishes between the following 

 types of elaiosome : — 



