414 THE BIOLOGY OF FLOWERING PLANTS 



of the burdock are similarly hooked. The seeds of some 

 species of Juncus have a mucilaginous coat which sticks 

 to the feathers of aquatic birds. Many birds may carry 

 small seeds in mud on their feet or feathers. Birds espe- 

 cially may thus be responsible for transporting seeds over 

 very long distances. Guppy (1917) puts down the dispersal 

 of European species of Juncus and Luzula to the Azores, 

 to the carriage of their sticky seeds on birds' feathers. 

 Nearly 20 per cent, of the species of the flora which has 

 developed on the island of Krakatau since the eruption in 

 1883, have, according to Ernst (1908), been transported by 

 birds. The distance to the nearest island not affected by 

 the eruption is over 12 miles, and the distance to the Javan 

 coast is some 25 miles. 



B. A great many Seeds and Fruits are transported by the 

 Wind. — Many are suited to this mode of dispersal simply 

 by their minute size. Seeds of orchids and of many 

 Ericaceae are Uke very fine dust. An orchid seed may weigh 

 only one five-hundredth part of a milligramme. Plants 

 with such seeds were amongst the first colonists of Krakatau, 

 observed three years after the eruption. They must obvi- 

 ously remain in the air for long periods. It is of interest 

 that most epiphytes have seeds of this type. There is no 

 direct method of reaching a favourable station, but minute 

 size and enormous production mean that some of the crop 

 will be able to germinate on suitable trees. 



Many fruits and seeds have special " flying " organs. 

 These take the form of plumes of hair, or of wing-like 

 expansions. The latter are well seen in the fruits of the 

 maples and the ash ; in the lime the bract functions as a 

 float. Winged seeds in our native flora are those of the 

 pine and the birch. In many tropical trees the seed wings 

 are very large and beautiful, as in Bignonia. Studies 

 by Ridley (1905) on tropical species have shown that the 

 wing does not keep the seed or fruit long afloat ; it seldom 

 travels, even in a fair wind, more than two or three times the 

 height of the tree. Rapid spread of trees, which only fruit 

 after many years' growth, is not to be expected (Fig. 66). 



