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III. 



ON THE BUOTANCY OF THE SEEDS OF SOME BRITANNIC 



PLANTS. 



By E. LLOYD PRAEGER. 



[Bead Aphil 15. Published May 31, 1913.] 



In the present paper the term " seed " is used in its original and familiar 

 sense, namely, that which is sown ; in other words, the natural unit of 

 dispersal. This may consist of a seed proper, or of one or more seeds 

 enclosed in a dry or fleshy envelope, varying greatly in different species as 

 regards size and shape. 



The capacity of seeds for remaining afloat in water is one which has a 

 very important bearing on the subject of the dispersal and distribution of 

 plants. Seeds which can float for weeks or months, or even in certain 

 circumstances for a few days, may become widely spread by the agency of 

 rivers and of lakes. If, in addition, they are capable of resisting the 

 injurious effects of salt water, the power of floating becomes more important, 

 conferring on the species possessing such seeds the possibility of dissemination 

 across stretches of sea of greater or less extent. 



Historical Summary. 



The importance of the floating power of seeds in relation to the 

 geographical distribution of plants has been long recognized. Tlie large and 

 buoyant seeds and seed-vessels which strew the shores of tropical islands, 

 with their suggestion of rapid and easy colonization by the aid of sea- 

 currents, have been familiar to botanical travellers since the days of the 

 earlier voyagers. So long ago as 1695, Sir Hans Sloane drew attention to 

 foreign seeds thrown up by the sea on the shores of Scotland and Ireland' ; 

 these were the seeds of tropical plants, brought by the Gulf Stream from the 

 West Indies. 



Darwin was the first to show that but a small proportion of flowering plants 

 have seeds which float, although a large variety of seeds are not injured by 

 even prolonged immersion in sea-water. He also found that by thoroughly 

 drying certain seeds and seed-vessels, their floating power was materially 



1 Phil. Trans., xix., pp. 39S-400. 1696. 

 SCIENT. PROC. R.D.S., VOL. XIV., NO. III. 



