168 



THE AMEKICAN MONTHLY 



[September, 



list of species includes twenty-seven 

 living forms, among which are found 

 ten species of rhizopods and nine- 

 teen forms of diatoms. 



In the month of October, 1848, 

 Ehrenberg recounted his researches 

 on the microscopic matters carried in 

 the atmosphere. He found on the 

 top of the veterinary school of Berlin 

 Eunotia amphyoxis and Pinnularia 

 borealis, two species which he had al- 

 ready indicated as characteristic of 

 the dust of all parts of the world, 

 and of all elevations. 



By washing plums, purchased by 

 him in Berlin, he found these same 

 species in the water employed, and he 

 obtained thefti again from mosses 

 from Mozambique. Moss taken from 

 a wall at Beirout, in Syria, fur- 

 nished him with the same species, as 

 did also some moss that he had taken 

 from the famous cedars of Lebanon 

 in 1820. He found them again on the 

 summit of the towers of " la place 

 des Gendarmes," Berlin. 



In many instances these diatoms 

 are specially mentioned by Ehren- 

 berg as living, that is to say, retain- 

 ing their endochrome, and in process 

 of division. In his work published 

 in 187 1, Ehrenberg recalled the fact 

 of a fall of meteoric dust at Calabre 

 in 1813, which contained specimens 

 dried during life at the time of multi- 

 plication. The dust which fell at 

 Lyons in 1846, contained them with 

 the endochrome still green. Since 

 that time Eunotia {Nitzschia) and 

 Pinnularia borealis, with colored 

 contents, have been frequently found 

 out of the water. In the dust col- 

 lected in 1834 on the Russo-Chinese 

 frontier, in 1844 at Quito, in 1848 in 

 lower Silesia and in divers other lo- 

 calities this fact has been made 

 known. 



The celebrated micrographer in- 

 quired, in one of his later publica- 

 tions, but without being able to 

 answer, how it happens that among 

 the four hundred species of diatoms 

 known by him in the vicinity of Ber- 

 lin, it is possible that two species 



among the most common of those 



found in the atmospheric dust, and 



which are also those one finds most 



frequently in the dust that deposits 



in Berlin, should be of the greatest 



rarity in the living state at the level 



of the ground 



********** 



In conclusion, I believe that the 

 diatoms mentioned above, and pro- 

 bably a considerable number of 

 others, should be considered as habit- 

 ually living on trees and in other 

 places exposed to the atmospheric 

 vicissitudes, particularly hygrometric. 

 I therefore recommend to my col- 

 leagues the methodical washing of 

 mosses, with a view of preparing a 

 list of the bacillaria, which they con- 

 tain, in a living state. The presence 

 of these species in tree-mosses easily 

 explains the fact of their presence in 

 the atmospheric dust, when they are 

 not found in the neighboring waters. 

 This is the solution of the question 

 which Ehrenberg proposed to him- 

 self without being able to resolve it. 



It is quite possible that these dia- 

 toms are among those in which the 

 phenomena of division and of conju- 

 gation and formation of spores are 

 the most active, rapid and easy to 

 follow, for these divers acts of life 

 among them must depend upon 

 showers or occasional rains, and cease 

 at the return of dry weather. The 

 study of these species will, without 

 doubt, serve to corroborate the inter- 

 esting observations of our colleague, 

 Paul Pettit, on the revivification of 

 diatoms. 



It is especially from these physio- 

 logical points of view that the study 

 of terrestrial diatoms merits our con- 

 sideration. 



[We have omitted the lists of dia- 

 toms given in the original article, but 

 the more common forms of terrestrial 

 diatoms, besides those already men- 

 tioned, belong to the following gen- 

 era : Eunotia, Navicula, Orthosira, 

 Nitzschia, Amphora, Pinnularia, 

 Stauroneis and a few others. — Ed.] 

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