12 Dust and its Importance to Plant Life. 



Besides this organic dust, rain-water contains ammonia and nitric 

 acid, said to be formed by electric discharges in the atmosphere.* 



At Rothamsted it has been calculated that in a single year 

 at least 3.971 lbs. of this electric nitrogen and about 1.3 lbs. of 

 organic nitrogen (apparently dust particles), or about 5 lbs. of 

 nitrogen in all, is deposited upon one acre.f 



Dr Reinke, in the Vosges, found that a litre of rain-water 

 contained .2 mg. of nitric acid and from .6 to 1.83 mg. of 

 ammonia. He estimates that in that district a hectare receives 

 about 2.5 kilogrammes of nitrogen per annum from the rain- 

 water. J This estimate is very close to that calculated for 

 Rothamsted. When rain-water falls upon the foliage it is not 

 suffered to remain upon the ordinary green surface of the leaves. 

 Dust would not only intercept the Hght but clog the stomata, so 

 seriously interfering with assimilation, transpiration, and respira- 

 tion. The water is therefore at once drained off by a system of 

 grooves and channels, which are often designed in a very perfect 

 way for this special purpose. 



Now one must bear in mind that this dust, considered as a 

 fertiliser, is exceedingly valuable. Fungus spores and algal cells 

 are packed with everything which is essential for the growth of 

 those plants. They are as the " tea cup " to the " ox " in a well- 

 known advertisement. In order to satisfy myself on this point, I 

 collected samples of dust from sixteen plants and found an extra- 

 ordinary proportion to consist of spores and cells. 



I did not proceed further with this examination, for it was 

 obviously true that mould fungus spores were exceedingly common 

 in such dust as well as bacteria, algal cells, lichensoredia, yeast 

 cells, and the .sjiores of Rust§ and other fungi. Moreover, 

 Hansen, Saito, and others had already shown that this was the 

 case both in Denmark and in Japan. 



I must next refer to an aged controversy in botany which has 

 l)een conducted, usually in a very violent manner, for at least 180 

 years. Can water containing dissolved nitr-ogeneous and other 

 salts be absorbed by the foliage? The celebrated liebig was, of 

 course, incorrect in supposing that flowering plants obtained most 



* Hoar fiost also deposits nitrogenous salts. — Anon., 1904. 

 t Hall. t Reinke. 



§ Klehalin. 



