MISCELLANEOUS PLANTS 



169 



and Australia. How comes it, we may ask, that the xerophytes 

 with which it grows in those three continents are confined in the 

 main to their respective regions whilst this plant occurs in all of 

 them ? What advantage can this species possess over all the hundreds 

 of Australian Acacias that have never wandered far from their 

 home? It is not merely that it is able to adapt itself to a littoral 

 station; for most xerophilous Acacias could do that; but it is able 

 to maintain itself there. A shore station gives it some special 

 advantage over the numerous xerophytic plants of the arid inland 

 regions. They come and go on the strands of all the continents, 

 but Acacia farnesiana remains. Why is that ? 



It is a matter of dispersal. The number of plants that are dis- 

 persed by currents, as I have shown in the eighth chapter of my 

 work on Plant Dispersal, must be very small indeed, almost infini- 

 tesimal in relation to the totality of species in the plant world. 

 From a somewhat extensive acquaintance with the buoyant capacities 

 of seeds and fruits 1 feel on safe ground in assuming that nearly all 

 the species of the chaparral scrub that reach the coast, as on the 

 Gulf shores of Texas and Mexico, possess no means of effective 

 dispersal by the currents. They may hold their own in places by 

 force of numbers, but they cannot extend their range along the 

 coast to localities where the chaparral is absent. In this respect 

 Acacia farnesiana possesses a great advantage, as I found in Hawaii, 

 since its indehiscent pods can float for a month unharmed in sea- 

 water. Its maintenance at the coast does not depend on recruits 

 from the inland scrub. It is ensured by the distribution of its 

 fruits by the currents. Though the seeds themselves sink, they 

 are buoyed up by the pod. 



A reference may here be made to my experiments in Hawaii, as 

 the details are not given in my previous work, where only the results 

 are stated. The moist green pods either sink at once in sea-water, 

 or float heavily and sink in a few days. They are entirely filled 

 with a kind of pith, the seeds having not yet accomplished their 

 hardening and shrinking stage. When the pod is ready to fall from 

 the tree, it is blackened and more or less air-dried, and the seeds 

 rattle freely inside. Its prolonged buoyancy is due to the cavities 

 produced by the shrinking of the seeds and the drying up of the 

 pith. Of five of these dry pods placed in sea-water, one sank in 

 sixteen days, the next in twenty-three days, and the others in from 

 thirty-two to thirty-six days, the cause of the sinking arising from 

 the decomposition of the pith and the gaping of the valves pro- 

 duced by the penetration of sea- water. This is far, however, from 

 the type of buoyancy one associates with fitness for trans-oceanic 

 dispersal by currents ; but it is well adapted for inter-island dispersal 

 in an archipelago and for extension along a continental sea border. 



The behaviour of the plant in the Hawaiian Islands is very sugges- 

 tive. Regarded by Hillebrand as of early introduction it has spread 

 over all the islands, and in places forms extensive coastal thickets. 

 Cattle spread the seeds over an island, and they may often be seen 

 germinating in their dung; but the currents accomplish the inter- 

 island dispersal. On some parts of Oahu, where the shrub grows 



