FLOATING MATTER OF THE AIR. 17 



The greater part of the floating matter of the air is 

 organic, for the heavier mineral dust is winnowed from it, 

 and falls first to the ground. Even the organic matter, 

 light as it is, would fall to the ground also, if the air were 

 perfectly still, which, of course, it never is in Nature. 



This floating dust consists of ground-up straw and rags,- 

 smoke, seeds, pollen, spores, germs, &c., which invade both 

 air and water to such an extent, that neither, however pure, 

 is entirely free from them (Figs. 6 and 7). 



The pollen or fine dust contained in the anthers of 

 flowers and catkins is often wafted into the air in thick 

 clouds in the neighbourhood of pine-forests, where it is 

 known by the popular name of " showers of sulphur." 



This is the dust with which the bees powder their coats 

 as they make their way in and out of the blossoms ; and so 

 fine and light is it that much is scattered in this way, and 

 floats away in the air. Then there are the spores which, in 

 flowerless plants such as ferns, lichens, and fungi, take the 

 place and answer the purpose of seeds. At the back of the 

 common polypody fern may be seen a number of little 

 round yellow dots, which are often called seeds, though they 

 are not even spores, but spore-cases. Each dot consists of 

 fifty spore-cases, and each case contains thousands of micro- 

 scopic spores, which, when perfectly ripe and dust-like, are 

 set free to be carried hither and thither by the air. 



Then there are the fungus-spores, the largest of which 

 are invisible without the aid of a microscope. These, since 

 they make up in numbers for what they lack" in size, are 

 constantly suspended in the air in large quantities, ready to 

 take possession of any suitable soil ; in proof of which we 

 c 



