Io6 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



disseminules. The lightest and most effective of these bodies, on 

 the other hand, come to grief chiefly through the action of moisture 

 which clogs their ' flying apparatus ', or condenses on them and 

 weighs them down. In this connection rain is extraordinarily 

 effective in removing, often during a single shower, practically all 

 of even the lightest spores, pollen grains, etc., from the atmosphere 

 through which it falls. For this reason, and because the strongest 

 and most lasting winds are chiefly at high altitudes, it is mainly 

 those botanical particles which reach the upper air which travel 

 really great distances. The fact that many do so appears to be 

 primarily due to the upward air currents resultant on the warming 

 of dark land-surfaces by radiant energy absorbed from sunlight. 

 Finally, for effective migration the plant has to become established, 

 and to that requisite any lack of suitable climatic or edaphic or 

 other conditions constitutes an insuperable barrier. 



Dispersal by Water and Ice 



The earliest forms of plant life were probably aquatic and water- 

 dispersed, and water still plays a very important part in the dispersal 

 of plants — particularly of those that live in or near it. But although 

 modifications that appear to be for water dispersal are found in a 

 wide variety of land plants, they are not so striking, or so widely 

 necessary, as those for wind dispersal. For practically any light 

 disseminule may be effectively dispersed by water up to the limit 

 of its ability to float and retain the power of germination — that is, 

 until it becomes waterlogged and sinks or decays, or until it is 

 killed, or, having begun to germinate, has failed to reach a suitable 

 habitat. Hence the main requirements for water dispersal are 

 sufficient buoyancy and impermeability, their degree of development 

 in a particular disseminule being often the most important factor 

 determining its success. 



Among Algae and many higher plants (such as the Canadian 

 Water-weed, Elodea canadensis) which normally live submerged in 

 water, there is no need for impermeability : the plant or special 

 disseminule merely drifts with any water current, sometimes attached 

 to floating logs, etc. Such drifting appears to be the main mode 

 of distribution of most seaweeds. Free-floating plants such as 

 Duckweeds [Lemna spp.) or Water Crowfoots are widely dispersed 

 as they float on the surface of the water, though they may sink to 

 the bottom to perennate. A fine tropical example is the Water- 



