10 Dust and its Importance to Plant Life. 



thing which may be of value is never lost, and this rule applies 

 even to these apparently lost pollen grains. 



For in the snow solitudes of the Alps there is an interesting 

 series of organisms contrived to make the best of this wandered 

 pollen, and one specially adapted to live in these cold and incle- 

 ment altitudes. Not only the snow alga (Chlamydomonas nivalis) 

 but a special insect and a snow mould fungus will together utilise 

 all the nitrogeneous and other material which will again reappear 

 as rich fertilising silt in the water of some small Alpine rivulet. 



The stigonemas and lichens which cover rock precipices and 

 pinnacles depend for their foodsalts upon chance pollen and 

 similar stuff brought by the rain and wind. 



In the lakes also the annual harvests of plankton algae and of 

 the minute animals which feed on them cannot be uninfluenced 

 by the masses of pollen which decay in the water. 



But my object to-night is to trace what happens to those 

 pollen grains which fall upon the foliage of other plants. It is, of 

 course, unnecessary to confine myself to the pollen, which is only 

 part, though by no means a negligible constituent, of atmospheric 

 dust. 



This dust is of a most interesting dix'ersity. Part of it im- 

 doubtedly once belonged to other worlds than ours, having, 

 perhaps, followed in a comet's train or formed part of some 

 shattered planet. Other dust particles have been extracted from 

 the earth's interior by the great volcanic eruptions of South 

 America or Japan. Deserts, ordinary sand dunes, and motorcars 

 also furnish contingents to it. I have myself seen a steamer's 

 deck covered with fine dust from the Sahara although we were 

 many miles from the African coast. The usual trades' dust is, 

 however, composed of minute algse (Tricho derma Hildebrandtii 

 var atlantica), allied to the form which is responsible for the name 

 of the Red Sea.* 



The household fires of Glasgow and of other great cities 

 contribute many carbon and other particles which have been 

 detected even on mountains several miles away.f 



But, wherever it has been examined, a very large proportion 

 of the dust is found to be of animal or vegetable origin. The 

 ingenious researches of Pasteur, Miguel, Hansen, Aitken, and 



* Reinsch, 1904. t Aitken. 



