THE BUOYANCY OF SEEDS AND SEEDVESSELS 



95 



In the sinking or non-hioyant group, which includes 8o per cent, 

 of the xvJiole, the mean specific weight is considerably greater than that 

 of sea-water {i'026), which zvoiild require its density to be raised to 

 riOO in order to sei^e as a floating medium for many of them. 



In the buoyant group the mean specific weight is much lighter than 

 that of fresh water (rooo) ; and from this it is to be inferred that in 

 oceans of fresh water the same fruits and seeds in the mass would be 

 distributed by the currents that are transported by them at the present 

 day. Even though it arose from an ocean of fresh water, the coral 

 island would receive the same littoral plants throtigh the agency of the 

 currents that it receives under its existing conditions. 



The number of plants with seeds or fruits between fresh water 

 and sea-water in specific weight is very small, probably not over 2 

 per cent, of the total. Most seeds or fruits that sink in fresh water 

 sink also in sea- water, and most that float in sea-water float also in 

 fresh water. Nature has thus created a zvide gap between the sinking 

 and the floating seed ; and nearly all of the work of the present 

 currents in pla7it-dispersal might have been effected, so far as the 

 density is concerned, in fresh zvater. She has not arranged seeds and 

 seedvessels in what the statistician would term " a good series." As 

 indicated in the diagram below, there are two series that meet in 



the neutral region where the density is between fresh water and 

 sea-water, but with culminating points placed on the one side far 

 above the density of sea-water and on the other far below that of 

 fresh water. 



