252 PLANTS, SEEDS, AND CURRENTS 



it on the beach. In the delta of the Mississippi it grows in great 

 luxuriance, giving its character and name to the plant-association 

 that clothes the higher portions of the alluvial banks between the 

 " passes " or mouths of that river : here, clambering amongst the 

 canes of Phragmites communis it forms an almost impenetrable 

 thicket (Harshberger, Phyt. Survey, N. America, pp. 216, 444, 

 under Vigna glabra, a synonym). 



The seeds of Vigna luteola, which are rather smaller than those 

 of V. lutea, float for a long time in sea- water. Some, which had been 

 seven weeks afloat in one of my experiments, germinated freely 

 afterwards. They help to form the smaller drift of beaches, as I 

 noticed at Trinidad. Amongst the floating drift of the estuary of 

 the Guayas River in Ecuador, I collected a number of sound seeds 

 of a species of Vigna, apparently those of V. luteola. 



The cause of the floating capacity of the seeds of both Vigna lutea 

 and V. luteola lies in a large central cavity between the cotyledons, 

 the materials composing the seed having no independent buoyancy, 

 a matter dealt with for the first species in my previous book (p. 106). 

 The genus holds about fifty known species, spread over the warm 

 regions of the Old and New World; and it would be important to 

 determine whether it follows the rule laid down in my earlier work, 

 that when a genus possesses both littoral and inland species only 

 the seeds or fruits of the shore species float. In the case of the 

 Hawaiian representatives of the genus the possibility was there 

 pointed out (p. 139) that the two endemic inland species were de- 

 rived from the coast species (V. lutea). The genus, it may be added, 

 offers many interesting problems for the consideration of the student 

 of distribution. 



XlMENIA AMERICANA, L. 



The writer made the acquaintance of this shrub or tree amongst 

 the littoral plants of Fiji, where its means of dispersal were investi- 

 gated, the results being given in his previous work on the Pacific. 

 Like the species of Scazvola and Cassytha that figure in the strand 

 floras all round the tropics, it can be dispersed in two ways, by birds 

 and by currents. Its drupaceous fruits are known to be distributed 

 by fruit-pigeons (Chall. Bot., L, 46); whilst the stones are able, as 

 I ascertained, to float in sea- water for months. But since the fruits 

 were rarely represented in the beach-drift, it was assumed that bird 

 agency has been predominant in the Pacific (Plant Dispersal, p. 113). 



Although, as Hemsley observes (Chall. Bot., IV., 132), it is a mari- 

 time shrub throughout the tropics of both hemispheres, it may 

 extend inland — a behaviour which it most frequently exhibits in the 

 New World. It has been recorded from Tahiti, Samoa, Tonga, 

 Fiji, from the islands of the Western Pacific, from North Australia, 

 Malaya, both coasts of Africa, and both coasts of the New World. 

 Its usual place among the trees and shrubs lining the beaches of the 

 Old World seems often to be abandoned, as just remarked, for an 

 inland station in the tropics of America. Though it is frequent 

 in the interior of South Florida, it has not established itself amongst 

 the characteristic littoral vegetation of the coasts or of the sand-keys. 



