1892.] on the Surface-Film of Water, &c. 549 



surface of the water, and you see in the lantern how they are 

 attracted to one another, point to point, and how they form long 

 chains, which have a tendency to break up into stars. It is the exist- 

 ence of such points of attraction on the margin of the leaves which 

 causes the duckweed to form chains and strings, so long as there is 

 any unoccupied surface in the pond. A moment's consideration shows 

 how profitable this tendency is to the plant. Were the duckweed to 

 crowd together like the floating bits of cork, the pressure towards the 

 centre of any considerable mass of plants would be so great that the 

 new leaves budded out would find no room in which to expand : but, 

 by virtue of one very simple provision, viz. the existence of inequal- 

 ities of level among the edges of the leaves, clear spaces and lanes are 

 left between the floating leaves, so long as any unoccupied space 

 remains. 



Long exposure to the air, especially in still weather, afiects the 

 life of duckweed in a material way. Dust and decaying organic 

 substances give rise to a pellicle, which is most mischievous to 

 floating plants ; and I think I could show, if time allowed, how 

 much the habits of duckweed have been altered thereby. But, apart 

 from visible impurities, mere exposure to air gives, as Lord Eay- 

 leigh has taught us, a considerable degree of superficial viscosity 

 to water. Hence, the leaves of duckweed, when the surface is 

 contaminated, will tend to lie in whatever positions they may be 

 thrown by accidental causes, such as wind, and the attractions due 

 to capillarity will be more or less impeded. But the effect of the 

 superficial viscosity will in time be overcome by the attractive 

 forces, so that it probably does not in the long run greatly affect 

 the distribution of the leaves over the surface of water. 



Many other floating plants, but not all, behave more or less like 

 duckweed, and for the same reason. As yet I know of none which 

 space themselves quite so effectually, and the extreme abundance 

 of the common duckweed, as well as its world-wide distribution, 

 may be partly due to the completeness of its adaptation to capillary 

 forces. Some dead objects may accidentally take a shape which 

 causes them to spread out over water, but I have met with none 

 which have particularly struck me. Floating natural objects, such 

 as sticks or seeds, behave, in many, cases at least, very differently, 

 and become densely massed. My attention was first called to this 

 subject by seeing how different was the grouping of duckweed from 

 that of some seeds of Potamogeton natans, which were floating in 

 the same pond. 



The capillary forces which spread the leaves of duckweed or 

 Azolla upon the surface of the water are indirectly concerned in 

 the transport of these and like plants to fresh sites. If we put a 

 stick into water overspread with duckweed, we cannot fail to notice 

 how the leaves cling to the stick. They cling in a particular way, 

 which enables them to bear transport more safely. The wetted surface, 

 for obvious physical reasons, is attracted to the wetted stick ; and the 



