IX THE GERMINATION OF FLOATING SEEDS 83 



the exception of one that did not swell and sink until after ten 

 months. On the other hand, in my experiment in Fiji on the fresh 

 seeds of Canavalia obtusifolia, a plant found on tropical beaches all 

 round the globe, seventy per cent, sank in the first six or seven 

 weeks, swelling and displaying the first signs of germination, but 

 quite ten per cent, were afloat after three months. 



My experiments on the foregoing and other littoral species of 

 the Leguminosae merely indicate that under the ordinary tempera- 

 ture of tropical currents a portion of the seeds will probably sink 

 owing to abortive attempts at germination. It is likely that if in 

 the experiments in England a constant temperature of 85° to 90° F. 

 had been sustained throughout, most if not all of the seeds would 

 have swelled and sunk within a month or two. The temperature 

 of the experiments in Fiji and Hawaii did not exceed that of many 

 tropical currents; but there are areas of superheating in equatorial 

 seas, which I think would prove insurmountable barriers in the path 

 of most drifting Leguminous seeds, a subject to which further 

 reference will be made. 



Coming to the Convolvulacese, my experiments show that the 

 buoyant seeds often lose their floating powers from the same cause. 

 Those of Ipomea pes caprse may be taken as an example. I was 

 surprised to find when experimenting on the buoyancy in sea- 

 water of these seeds in Fiji and Hawaii that a considerable pro- 

 portion, about a third, sank in the first two months, swelling and 

 sinking to the bottom. That this swelling represented the early 

 stage of germination was well brought out in parallel experiments 

 in fresh water and sea-water made in England on the buoyant 

 seeds of the British littoral species. Convolvulus soldanella. A 

 good proportion of the seeds in the first part of the experiment 

 absorbed water, swelled, and sank, those in fresh water proceeding 

 at once to germinate healthily at the bottom, whilst those that sank 

 in sea-water merely decayed. Of the survivors about fifty per 

 cent, in either case floated after six months. It may be added 

 that the seeds of other tropical littoral species, such as those of 

 Ipomea glaberrima and I. grandiflora, behaved in the same way. 



It would appear from my experiments, and it is a result that 

 we should expect, that buoyant seeds of the Leguminosae and 

 Convolvulaceae would often float for much longer periods under 

 cool than under warm conditions. There must be areas of high 

 temperature in mid-ocean that would prove much more fatal to the 

 chances of a drifting tropical seed than the icy waters of a Polar 

 current. In my paper on Keeling Atoll I have described how I 



G 2 



