I 9 8 ROUND THE YEAR 



weed. They will be equally attracted by the sides of 

 the vessel, unless this is made over-full of water. 



To get direct evidence of the existence of centres of 

 attraction around the floating fronds, set one floating 

 upon water, and bring near it a clean glass rod dipped 

 into the water. There will be an ascending capillary 

 curve around the rod, and this will attract the frond, 

 which will turn itself about, so as to bring one of its 

 centres of attraction next to the rod. The frond may 

 be dragged about the surface and made to turn round 

 without being touched. 



What good does the Duckweed get from these 

 centres of attraction ? It is these which cause the 

 fronds to cohere into strings and chains instead of 

 forming a compact mass. A moment's thought shows 

 how profitable this is to the plant. Were the Duck- 

 weed to crowd together, like bits of cork or seeds, the 

 fronds in the centre of the mass would be unable to get 

 room for budding. It would be necessary to displace 

 a vast number of mutually attractive bodies before a 

 single new frond could be pushed out. But by the 

 simple provision of inequalities of level along the 

 margin, the fronds group themselves in stars and 

 strings, with lanes between them, so that they can 

 push forth fresh buds without difficulty as long as any 

 unoccupied space remains. 



The same capillary forces aid in the transport of 

 Duckweed to fresh sites. If we put a stick into water 

 overspread with Duckweed, we cannot fail to notice 

 how the fronds cling to the stick. They cling in a 

 particular way, which enables them to bear transport 

 more easily. The wetted surface of the frond is 



