I08 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



of inundated areas. Such dispersal is, however, virtually limited 

 to the direction of the current and to the particular land-mass con- 

 cerned, the disseminules of other than marine and strand plants 

 rarely surviving protracted flotation in the ocean. Thus all manner 

 of seeds, fruits, and living fragments of aquatic or river-bank plants 

 are to be seen among the ' flotsam ' of debris floating downstream, 

 often to be left stranded in situations suitable for growth and 

 establishment, while in tidal estuaries migration is often away from 

 the mouth of a river, aided by tides which run upstream as well as 

 down. The ebb of a high tide where the water is brimming widely 

 is particularly efi^ective in the deposition of floating materials. 

 Examples of flowering plants regularly dispersed by freshwater 

 streams are many of the Pondweeds [Potamogeton spp.) whose small 

 fruits in some instances can float for months on end, and the Yellow 

 Water-lily {Nuphar hitea), the pulpy fruit of which floats for a few 

 days before disintegrating and releasing the seeds, which sink and 

 later germinate. An example of a species whose seeds, as such, are 

 commonly distributed by water, is the Summer Snowflake {Leiicojum 

 aestiviim). Casually, almost any plant or its disseminules may be 

 transported downstream by flotation, striking examples being the 

 alpine species that are often to be found in open streamside habitats 

 in the lowlands. Familiar instances in the boreal regions are such 

 ' open-soil ' types as Mountain Sorrel {Oxyria digyna), Moss 

 Campion {Silene acaulis agg.), and various Saxifrages. 



{c) Rainwash, floods, and lakes. Rain not only splashes out the 

 seeds or spores from open organs but, when forming a wash, may 

 carry them much farther than other agencies commonly do — 

 especially when it develops into a flush or extensive run-off, perhaps 

 in time to form a rivulet or even to join a major stream. A con- 

 siderable run-off may be noted in boreal regions when the snow 

 melts in spring but the ground remains frozen and impervious. 

 Often it is not necessary that disseminules, in order to be washed 

 away, should be able to float, though to reap the benefit of wider 

 dispersal by ordinary floods they should do so, as of course they must 

 normally do to be blown about on lakes. Almost any plant or part 

 of a plant may in certain circumstances be dispersed by drastic 

 floods, involving as they do the uprooting of trees and the carriage 

 of all manner of debris, sometimes for considerable distances — per- 

 haps to be deposited in a silty flood-plain well suited to the establish- 

 ment of migrant plants. In lakes the methods and plants involved 

 are in general similar to those in streams, but there is more limitation 



