Dust and its Importance to Plant Life. 11 



Saito have also shown that the " motes ' ' existing in it are always 

 changing. Even when two flasks are placed side by side and their 

 air contents examined at the same time, the results may be utterly 

 different. One flask may contain many motes and germs, whilst 

 the other has a very small number, and there may not be any 

 similarity in the character of the motes themselves. 



It is possibly safe to say that for three or four thousand feet 

 above our heads the air is full of motes, and especially of spores ; 

 some are floating by themselves, others are entangled in " clouds." 

 They are all apparently moving not only because they do not 

 quite travel with the earth's rotation, but because bodies so light 

 and minute are at once set in motion by any the slightest current 

 or eddy in the air and by the very faintest breath of wind. 



There is a continual supply of such spores and of germs. A 

 piece of decaying material, say, for instance, the drying sputum 

 of a consumptive person, if left upon the street, will give off 

 bacteria which may be carried up and in at a fifth storey window. 



Professor Cieslar has shown that the spores of the larch 

 disease fungus may be blown up by air currents to a height of 

 sixty feet above the earth. Yet no such estimates are at all 

 reliable, for, when once launched in the atmosphere, there is no 

 assignable limit to the voyaging of a dust particle, for it might 

 quite well travel three times round the earth and come to rest 

 anywhere upon its surface. 



Many curious facts of distribution are best explained by these 

 considerations. Thus the snow alga mentioned above occurs not 

 only on the Alps but on the Andes and even on Ruwenzori.* 

 Certain mosses and lichens are found not only in the Arctic but 

 also in the Antarctic region, and though there are a few lonely 

 mountain summits between the North and South Pole on which 

 they may occur, yet there are wide intervals over which they were 

 probably carried by the wind or possiblv bv birds, t 



When rain is about to fall the drops condense upon a dust 

 particle and carry down with them many others whilst falling. 



* Roccati, 1909. 



t Even such relatively substantial bodies as seeds are sometimes 

 carried by wind. Plants with dust-like seed are relatively 

 very common on islands far out at sea. On Christmas Island 

 33 plants (out of 170) have dust-like seeds.— Chardot, Ridley, 

 1905. 



