4] DISPERSAL AND MIGRATION IO9 



of effective dispersal to aquatic and semi-aquatic types, and the 

 distances of dispersal are usually small. Most often, partially corky 

 or other air-containing tissues cause the body involved to float, or 

 buoyant vegetative parts are detached by feeding Mammals or 

 wildfowl. 



Fig. 26 shows a range of water-dispersed bodies. 



(d) Icebergs, ice-floes, etc. The ' rafting ' of all manner of 

 material, including living plants and their disseminules, after blow- 

 ing, falling, or spring-time washing on to fast-ice near the shore 

 or on to glaciers which later ' calve ' to form icebergs, has been 

 widely recognized in arctic and subarctic regions. There can be no 

 doubt that by this means much material is transported out to sea 

 and often far away before the ice melts and releases it, though it 

 seems unlikely that the disseminules of land plants find their way 

 back to suitable habitats at all frequently. Probably more important, 

 and certainly more frequent, is the dispersal of Diatoms, particularly, 

 which grow upon the ice-floes and may in time travel hundreds or 

 even thousands of miles with them, or of such strand-plants as 

 Creeping Alkali-grass {Piiccinellia phryganodes agg.) which are ' picked 

 up ' after being frozen solid in ice that forms about the shores on 

 which thev grow. It mav be presumed that these occurrences were 

 more widespread during the Ice Ages, though there are instances 

 occurring even well south nowadays — e.g. in the estuaries of the 

 Atlantic seaboard of the United States. Ice floating down rivers or 

 blown about lakes may also be of significance in carrying disseminules 

 that do not float. The present writer has investigated the plant 

 materials collected on a large ice-island in the vicinity of the North 

 Pole, that had drifted many hundreds of miles from the point 

 where they were washed or blown down from the land on which 

 they grew. Almost all of these materials were dead, but those 

 collected when the ice-island had drifted at the very least 3,000 miles, 

 and quite possibly several times that distance, included an extensive 

 though thin tussock of the Moss Hygrohypnum polare which was 

 found to be still alive. 



Charles Darwin, in The Origin of Species (6th edn. 1873, P- S^^), 

 after noting that the natives of the coral islands in the Pacific procure 

 stones for their tools solely from the roots of drifted trees, remarked 

 that these roots also frequently enclose small parcels of earth ' so 

 perfectly that not a particle could be washed away during the longest 

 transport : out of one small portion of earth thus completely enclosed 

 by the roots of an oak about 50 years old, three dicotyledonous plants 



E 



