4] DISPERSAL AND MIGRATION gg 



Wind Dispersal 



A walk in the woods and fields of a north-temperate region on 

 a boisterous autumn day should convince any sceptic that air currents 

 of one kind or another are important in the dispersal of many 

 different plants. Not only do winds blow leaves, and sometimes 

 small branches, about— and with them adhering parasites or sapro- 

 phytes, for example — but they obviously transport some seeds and 

 fruits for considerable distances. The more efficiently adapted of 

 these, whose bodies are so light or whose ' form-resistance ' is such 

 that they sink only slowly in still air and float almost indefinitely 

 in a light breeze, may be transported far from the parent. This 

 undoubtedly happens with such plumed seeds as those of Milkweeds 

 {Asclepias spp.) and Fireweed {Epilobium migustifolium agg.), or with 

 such ' parachute ' fruits as those of Dandelions {Taraxacum spp.) 

 and many other members of the Daisy family (Compositae). 



Even more effective is the dispersal by air currents — including 

 upward eddies that carry them into the upper atmosphere — of 

 microscopic spores, especially of Fungi and Bacteria. Such dis- 

 persal may take place over distances that in numerous instances 

 have been proved to run into many hundreds of miles. The present 

 writer has studied this subject for years and is convinced that these 

 smaller ' botanical particles ' or ' spora ' can be (and often are) 

 carried thousands of miles in the atmosphere, frequently without 

 losing their power to resume active life on regaining suitable con- 

 ditions. Thus he has trapped some spora in the immediate vicinity 

 of the North Pole under both winter and summer conditions, as well 

 as elsewhere at vast distances from their nearest conceivable point 

 of origin. 



Quite apart from disseminules which are specially modified for 

 transportation by winds, and others which are so minute that they 

 need not be so modified to be transported, there are many recorded 

 instances of large and heavy bodies being blown for considerable 

 distances by hurricanes, etc., on special occasions. After the 

 devastating tornado in and around Worcester, Massachusetts, in 

 June, 1953, abundant shingles and often bulkier roofing materials 

 and sizeable living branches of trees were to be seen littering the 

 ground fully 20 miles nearer the coast than the closest point at which 

 the ' twister ' had struck. There are also records of windfalls of 

 uprooted plants scattered over wide areas. It need scarcely be 

 remarked that, as successful transportation and growth of a single 



