THE ANGIOSPERMAE 1567 



The genus Pisonia (Nyctaginaceae) consists of trees and shrubs which 

 are found in all warm regions, usually near the seashore and frequently on 

 oceanic islands. The one-seeded fruits are covered by the intensely sticky 

 perianth tube which adheres firmly to birds' feathers. So glutinous are 

 the fruits that they sometimes trap smaller birds, who cannot disentangle 

 themselves. Even large birds such as Herons and Boobies can be seriously 

 troubled by them though it is chiefly by their agency that the plants are 

 distributed. 



The classic case of a sticky fruit is that of Mscum album, the Mistletoe, 

 whose berries are eaten by Thrushes. The glutinous pulp, with its con- 

 tained seeds, sticks to the bird's beak and is carried away. The thrush 

 then strops its beak on the bark of another tree, to clean oflt the mess, and 

 in so doing plants the seeds in crevices of the barkwhercthey later germinate. 



Simple adhesion in wet mud, without any special device, to the feet and 

 fur of animals and birds, or to the boots and wheels of mankind, is quite 

 common and affects a great many plants, especially those which grow in 

 wet places. Wading birds are very important in this kind of dispersal and 

 sea birds undoubtedly carry many seeds in this way to distant islands. 

 Nearly all the plants concerned are small herbs, trailers or scramblers 

 which may be trodden on, or else semi-aquatic plants among which water 

 birds may swim or wade, or animals like the Moose or the Water Buffalo 

 or Wild Boar may trample and wallow. In Pleistocene days when great 

 herds of grazing animals, Deer, Reindeer, Bison, etc., roamed the northern 

 countries, transport and distribution of plants by their means miust have 

 taken place on a great scale. 



Charles Darwin was the first to draw attention to the feet of birds as a 

 means of dispersal. From a ball of mud from the leg of a partridge he 

 raised twelve Monocotyledons and seventy-two Dicotyledons, of three 

 different species. Marsh birds fly direct from one marsh to another and 

 by their agency many, perhaps the majority, of marsh plants are carried 

 from place to place, even to very isolated spots. In this way plants are 

 frequently noticed appearing around ponds where they were previously 

 unknown, although the nearest locality for the species may be many miles 

 away. One of the authors has seen Damasonium alisma appear thus sud- 

 denly by a much-frequented pond in Berkshire, though the plant had been 

 unknown in the county for over a century. 



The W'heels of farm carts and muddy boots also act as carriers. One 

 may often see cart tracks filled with such plants as Radiola linoides, Junciis 

 biiforuiis, Montia rerna or Tillaea muscosa, affording evidence of dispersal 

 bv a combination of adhesion and rain wash. 



Dispersal by internal carriage is generally associated with fruits which 

 are pulpy and attractively coloured and flavoured, so that they are eaten 

 by birds or other animals. Occasionally it may be an aril or some part or 

 the flower, such as the calyx or the receptacle which provides the attraction 

 (Fig. 1430). The seeds or endocarps are hard and indigestible and they are 

 voided uninjured, or even softened and rendered more apt for germination. 



