Praeger — The Buoyancy of the Seeds of some Britannic Plants. 53 



greatly diminished by separating the pappus from the aohene, the aehenes 

 sinking soon, while the complete fruit remained afloat. This is an effect of 

 the entanglement of air-bubbles in the pappus, as the following experiment 

 showed : — 



Twenty fruits of Taraxacum were divided above the achene, and the 

 aehenes and pappuses, as well as twenty more entire fruits, were immersed 

 in water, care being taken to expel from the pappus the bubble of air which 

 frequently lodges at the point whence the pnppus-liairs radiate. At the end 

 of twenty-four hours — 



Of 20 fruits, 17 had sunk. 



Of 20 aehenes, ..... 15 „ 

 Of 20 pappuses, . . . . . 20 „ 

 The three fruits which still floated were then divided above the 

 achene, whereupon their three pappuses sank at once. So that the total 

 result of 24 hours' immersion was the sinking of 40 pappuses and 

 32 aehenes. This shows that, wlien free of air-bubbles, the pappus hinders 

 instead of assisting the floating of tlie Dandelion fruit. Carduus lanceolatus, 

 Crepis biennis, and Sonchus arvensis, tested similarly, all gave a slightly 

 higher buoyancy for the achene than for the bubble-free pappus. But in 

 nature, an air-bubble frequently lodges at the base of the shuttlecock of 

 bristles, and maintains its position tenaciously ; so that in this way the 

 pappus may cause the fruit to float until the bubble is expelled by rough 

 treatment, or is dissolved in the water ; and it may thus materially aid the 



floating power of the fruit. 



Ericaceae. 



Species tested, 10. Buoyancy low. Erica vagans (3| days) and 



Pyrola minor (2| days) alone float for over a day. The dried fruits of 



Arctostaphylos Uva-tirsi float for a day or so, but those of Arbultis sink at 



once, as do its seeds and fresh fruit. 



Primulaceae. 



Species tested, 9. LysimacUa vulgaris, the only marsh-plant among 



them, alone possesses any buoyancy (one day in my samples, 1-4 weeks in 



Guppy's). Guppy adds L. thyrsiflora, witli a 1-4 weeks' buoyancy. 



Boraginaceae. 



Species tested, 10. Buoyancy variable and low, 4| days being the 



maximum (in Myosotis caespitosa). 



Orbanchaceae. 



The extremely light seeds of the Broom-rapes do not help them to float. 



Six species tested sank in between 1 hour and 4 days. 



SCIENT. PECO. B.D.S., VOL. XIV., NO. HI. H 



