DUCKWEED 197 



capillary curve around each. The result is that the 

 disc will be attracted to the side of the vessel. Every 

 time that it is moved away for a short distance it comes 

 back again. But if we add water little by little with 

 a syringe until the vessel becomes over-full, we convert 

 the ascending curve into a descending one. Unlike 

 curves are now brought together, and the disc is 

 repelled. By sucking up a little water the ascending 

 curve can be restored, and then the disc will be 

 attracted by the side. Thus we may go on as long as 

 we please, causing the disc to be attracted and repelled 

 by turns. 



The shape of one of the fronds of our commonest 

 Duckweed (Lemna minor) has already been described 

 (p. 193). At each end of the ridge which runs along 

 its upper surface the margin of the frond is slightly 

 raised above the water-level, and to it the water rises 

 in an ascending capillary curve. Each of these raised 

 parts of the margin will be a centre of attraction to a 

 like centre on another frond. The free edge of a 

 budding frond is also raised above the water-level, and 

 forms another centre of attraction. Hence, when a 

 number of fronds float upon water, they are attracted 

 to one another at certain points, while the intervening 

 parts of their margin come flush with the water and 

 are inert. We can imitate the effect by models. Cut 

 out of paper boat-shaped strips, say half an inch long, 

 and pointed at each end. Turn every point up, and 

 set the strips floating upon water. There will be an 

 ascending capillary curve at each end of every strip, 

 and these will attract one another, so that the strips 

 will arrange themselves in chains and stars like Duck- 



