112 A NATURALIST IN THE PACIFIC chap. 



final stage of maturation. A double cause is also to be assigned 

 to the buoyancy of the fruits of Heritiera littoralis and of Smythea 

 pacifica, where, in addition to the unoccupied space produced by 

 the shrinking of the seed, the fruit-case itself floats, though nothing 

 but a mechanical explanation is to be given of the floating of 

 empty ligneous fruits. 



One of the most suggestive types of buoyancy belonging to the 

 first group is presented by those cases, which are, however, not 

 very frequent, where the floating power is to be attributed to 

 empty seed-cavities produced by the abortion of the ovule or 

 failure of the development of the seed. A significant instance of 

 this is afforded by the fruits of Premna taitensis, a coast plant. 

 The buoyant " stone " of the drupe, which is often found afloat in 

 the Rewa estuary in Fiji, is 4-locular, each cell containing normally 

 one seed, but as a rule only one cavity contains a mature seed, the 

 three other cavities becoming more or less empty through the 

 failure of their seeds. It can be proved that neither the seeds nor 

 the substance of the " stone " are buoyant, and that the " stone " 

 owes its capacity of floating for months to the empty cavities 

 arising from the failure in development of three out of the four 

 seeds. In Fiji we see the rivers distributing these small fruits, and 

 we find the " stones " stranded on the beaches and floating in the 

 currents amongst the islands ; and there can be no doubt that this 

 is one of the effective modes of dispersal of the species ; yet, if 

 there was ever a case of accidental buoyancy concerned with 

 dispersal by currents, we have it here. Further details are given in 

 Note 32. 



It is probably also to the abortion of the ovule, or to the 

 failure of the seed, that the remarkable air-cavity (see Note 8) to 

 which the pyrenes of Morinda citrifolia owe their floating power, is 

 to be attributed. To this structure Professor Schimper (pp. 165, 

 183, 2(X)) attaches considerable importance as an example of 

 special adaptation to dispersal by currents through the influence of 

 Natural Selection. He suggests, however, that possibly its mor- 

 phological significance may be found in its being a peculiarly 

 modified seed-chamber. The case of Premna taitensis above cited 

 indicates that the latter view is the most probable. The subject 

 awaits a careful microscopical study of the seed-development of 

 the genus Morinda since, as elsewhere remarked, the non-buoyant 

 pyrenes of inland species have not such an air-chamber. An 

 outline sketch of a pyrene of Morinda citrifolia is given in the 

 preceding plate. A good figure of it occurs in Schimper's Plant 



