100 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



plant or disseminule is sufficient for its establishment in a new region, 

 even extremely rare occurrences may be important and involve 

 quite unexpected species and circumstances. Instances in point 

 include exceptional winds in various regions, and the blowing of 

 seeds, fruits, or whole plants over the ice or compacted snow in 

 arctic regions in winter. Not only may such blowing over ice be 

 effective from time to time, in the case of higher plants, but, more 

 often as they tend to remain longer alive, it may also result in the 

 dispersal of parasitic or saprophytic Fungi, etc., growing upon or 

 within the bodies of these higher plants. When we recall that, not 

 very many thousands of years ago, ice covered vast tracts of what 

 are now among the most populous parts of the northern hemisphere, 

 as well as, doubtless, the adjacent seas, we can imagine that such 

 dispersal may have been of great importance in the migrational 

 history of plants in areas far south of the present-day Arctic. 



It is instructive to consider briefly the main organs or methods 

 of wind-dispersal, and particularly those plant bodies which are 

 especially modified for the purpose. For this, there should be 

 recalled the distinction between seeds and fruits which was explained 

 on page 69, and the very different origin and nature of spores in 

 different cases. 



(a) Spores. These, as we saw in Chapter II, are the main dis- 

 seminules of most of the groups of plants up to and including the 

 Ferns, and are often produced in fantastically great numbers. Thus 

 a single specimen of the Pasture Mushroom {Ag^ricus {PsaUiota) 

 campestris) has been estimated to produce 1,800,000,000 spores, 

 while a large specimen of the Shaggy-mane Mushroom {Coprinus 

 comatus) may produce 5,240,000,000 spores, and some Puff balls 

 many times that number ! Although extremely variable in size 

 and form, spores are commonly minute and easily blown about by 

 the wind — being frequently borne by upward air-eddies rising from 

 warm plains and carried into the upper atmosphere where they may 

 be transported vast distances. Indeed, like volcanic dust, they are 

 probably sometimes blown around the world without settling to 

 earth. Bacterial and some other minute cells may belong to the 

 same category as spores in the matter of size and aerial buoyancy. 

 The spores are often extremely resistant to low temperatures and 

 desiccation which in fact appear to prolong their life, so that many 

 caught in the most remote situations are alive, able to germinate 

 when given suitable conditions, and, as we say, ' viable '. They 

 may live for many years and, apparentlv, often withstand the radia- 



