ECOLOGICAL 197 



In adaptation to this mode of dispersal there is often a resistant 

 outer coat which prevents the premature entrance of water, and 

 there is often a layer of air-cavities which secure flotation. The 

 Coco-nut can be carried far by oceanic currents, and is widely 

 distributed on inhabited islands in tropical seas ; its original home is 

 uncertain, but there is some evidence for the Magdalena Valley in 

 the north of S. America. There has been a tendency to exaggerate 

 the Coco-nut's adaptation to dispersal by water. The hard waxed 

 epicarp keeps the water out; the fibrous mesocarp makes the fruit 

 buoyant; the thick stone or endocarp prevents injury to the seed 

 when the fruit is battered about on the shore; the milk supplies 

 nutriment and fluid enough to keep the washed-up seed sprouting 

 until the root has penetrated below the surface salt into a freshwater 

 layer of soil. It must be admitted, however, that while Coco-nuts are 

 often found among the jetsam of oceanic islands, they are very 

 rarely known to sprout. They are not known to survive imless man 

 looks after them. Perhaps, however, successful sprouting may occur 

 when exceptional tides or storms land them well above the usual 

 high-tide mark. Probably, however, the adaptations are chiefly 

 useful to prevent injury to the delicate seed when the large and 

 heavy fruit falls from the tall tree. 



Familiar in greenhouses is a Madagascar aquatic plant Ouvirandra, 

 in which the leaves that float on the surface become ruptured in 

 growth by a multitude of meshes between the veins, and look like 

 skeleton leaves. In reality this effects a great increase in the surface 

 available for gaseous exchange. In Ouvirandra berneriana there is a 

 strikingly adaptive mode of dispersal. The fruit bursts explosively 

 and the buoyant seed floats on the surface, where it rapidly germin- 

 ates. As it gets free from its wrappings, it sinks to the floor of the 

 stream, and is at once ready to eflect root-attachment. This unusual 

 sprouting while free floating probably reduces the risk of being 

 washed away or driven on to the land. Many of the mangroves on 

 the tropical shore effect the same by being viviparous; that is to say, 

 the germination is effected while the fruit is still hanging on the 

 parent tree ; hence what drops on to the beach is a young plant able 

 to attach itself before it is swept out to sea. 



(3) The shrivelling of the carpels composing a dry seed-box, 

 analogous to the withering and fall of the foliage-leaves, allows the 

 seeds to tumble out; and this commonplace dehiscence leads on by 

 gradations to ruptures sometimes sensational in their explosiveness. 

 Everyone is familiar with the little popgun explosions of the broom 

 pods on a sunny autumn day, by which the seeds are catapulted out 

 to a distance of several feet. There is nothing vital in this, for it is 

 entirely due to the unequal shrinkage of dead cells in the wall of the 

 drying pod. But this is not the case when a touch sets free the five 

 valves of the balsam, whose expressive name of "Touch-me-not" 



