CHAPTER X 



THE RELATION OF THE BUOYANCY OF SEEDS AND SEED- 

 VESSELS TO THE DENSITY OF SEA-WATER 



The general principles concerned. The subject assumes a statistical character, 

 Seeds and seedvessels are as a rule either much heavier than sea-water 

 or much lighter than fresh water. The present littoral plants with buoyant 

 seeds or seedvessels could be equally well dispersed by currents in oceans 

 of fresh water. Seed-buoyancy has no relation either in the present or in 

 the past to the density of the sea. Though an accidental attribute, the 

 specific weight of seeds has had a profound influence on plant-distribution. 

 Summary. 



To find amongst the results of my numerous experiments 

 examples illustrating the influence of density on flotation has not 

 been so easy as I at first imagined. Excluding all adventitious 

 causes of buoyancy, a matter discussed in Note 40, it may be 

 inferred that the great majority of .seeds and fruits sink both in 

 fresh water and sea- water. Of those that are buoyant many float 

 indefinitely in both waters, whilst in a very few cases, where the 

 floating power is derived from an outer fleshy covering, as with the 

 fruits of Potamogeton natans, the fruits float a much shorter time 

 in sea-water than in fresh water, on account of the injurious effect 

 of the salt upon their coats. 



Experiments have to be specially directed towards this subject. 

 It would be useless to experiment in fresh water at one time and 

 in sea-water a month later. Nor would it answer to employ seeds 

 and fruits from different localities, since variations in this way 

 sometimes occur. It is necessary that the experiments should 

 be made on seeds or fruits collected at the same time and place, 

 and that they should be simultaneous and carried on under the 

 same conditions. As the discussion proceeds, the reader will 

 perceive that many interesting points are opened up, and that such 



