4] 



DISPERSAL AND MIGRATION 



105 



Fig. 25. — Wind-dispersal mechanisms and disseminules. A, capsule of an 

 Orchid {Cymhidiiun), open, with minute seeds being scattered by the wind ( < \); 

 B, fruit of Milkweed (Asclepias), showing liberation of the effectively plumed 

 seeds (X A); C, ' parachute ' fruit of Dandelion {Taraxacum) {:< 2); D, flattened 

 seed of Macrozanouia with large papery wing ( < -A); E, pollen grain of a Pine 

 (Piniis), with inflated, bladder-like ' wings ' making it buoyant in air ( ,■ 390) ; 

 F, flattened fruits of an Ash (Fra.xhuis) ( li); G, fruit of Maple (Acer) with 

 flattened wings (.; i); H, fruit of Linden (Lime-tree, Tilia), adapted for wind 

 dispersal by being attached to a specialized leaf (bract) ( < f); I, capsule of Poppy 

 (Papaver), from which the seeds are liberated only on \iolent shaking ( X 25). 



instances being repeated again and again. When such bodies alight 

 on even small tracts of water, these are apt to constitute insuperable 

 barriers to disseminules which cannot float for a protracted period. 

 Thus it has been observed that plants depending on winged seeds 

 or fruits for their dispersal are rare on oceanic islands. Dense 

 forests may have an effect similar to oceans, though of a less finite 

 nature. However, the fact that many disseminules await a parti- 

 cularly strong gust of wind before becoming detached from the parent, 

 is obviously advantageous in that such stronger winds are the more 

 likely to carry them afar. 



Mountain ranges also prove a barrier in many instances — though 

 the fighter disseminules are easily blown up and over them — as, 

 to a lesser degree, do cliffs, walls, and fences. This is evidenced 

 by the fact that beneath such obstacles a wide range of wind-borne 

 seeds and fruits are often to be found germinating, having been 

 stopped in their flight and fallen down. Pits and other depressions 

 have much the same effect in providing a barrier against the heavier 



