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coloured microbes, which can, if not assimilate, at least tolerate without injury the 

 full rate of organic matter and of nitrate-.and ammonia-nitrogen of fertile garden-soil. 



A high glasscylinder is filled for one half with garden-soil, for the other with 

 pure water. After shaking the thus obtained mud is allowed to stand at a sunny 

 window. After some days or weeks, according to season and temperature, one 

 sees at the illumined side of the glass a deep brown film appear, consisting of the 

 Diatoms present in the garden-soil, which slowly creep towards the light. This 

 film increases some months by the multiplication of the Diatoms, but finally there 

 appear in it large green spots of various lower Chlorophyceae, whose propagation 

 becomes only vigorous, when the Diatoms and other microbes (such as bacteria 

 and monads) have for the greater part used the assimilable organic substances and 

 converted them into unassimilable material. Cyanophyceae do not grow under these 

 circumstances, this being prevented by the abundance of nitrogen-compounds in the 

 garden-soil. 



Though it is certain, that the flora of Cyanophyceae in my tap- and canal-water 

 experiments only develops with an extremely small porportion of organic matter in 

 the food, I still consider this proportion to be of an essential signification for the 

 experiment. I have already convinced myself that at as complete an absence as 

 possible of organic substances, the development of the flora follows quite a different 

 course, but I am as yet unable thereabout to impart any decisive results. 



The experiment now described, is not quite new as to its principle. In another 

 form it was already performed in 1892 by Schlosing fils and Laurent 1 ), not 

 however with a culture liquid, but with a solid sand-soil and under conditions 

 much more complicated than mine. Noteworthy is that also these investigators, 

 cultivating in the light under the exclusion of all compounds of nitrogen, obtained 

 Cyanophyceae belonging to the same or almost the same genera as those resulting 

 from my experiments. They have moreover come to the result that by these 

 Cyanophyceae free nitrogen was assimilated in a slight but distinctly observable 

 quantity, and though they have not completely proved this assertion, as their 

 cultures must have contained other organisms too, e.g. many bacteria, basing also 

 on my own experiences I take their view to be correct. 



My experiment throws some light on the two folio wing observations. Graebner 2 ) 

 observed that fresh sandy grounds, which are changing into moors, cover in the 

 beginning with a flora of Cyanophyceae; and Treub 3 ), when visiting the isle of 

 Krakatau after its destruction, found that the new flora which first developed on the 

 volconic ashes, likewise consisted of Cyanophyceae, of which he in particular mentions 

 Lingbya verbeekiana and L. minutissima. Both, the said heathsand and the ashes of 

 Krakatau, have no doubt been extremely poor in nitrogen-compounds. 



If absolutely rejecting the theory of spontaneous generation, it might be 

 assumed that certain Cyanophyceae, carried over from the universe by meteorites, 



') Fixation de 1'Azote libre par les plantes.Ann.de 1'Institut Pasteur T. 6 pag. 832, 

 1892. The authors make special mention of Nostok punctiforme, N. mi-nut u in and Cy- 

 lindrospermum ma jus. 



-) Studien iiber die norddeutsche Heide. Bot. Jahrbiicher. Bd. 20, 1891. 



8 ) Notice sur la nouvelle flore de Krakatau. Ann. d. Jard. Bot. de Buitenzorg. 

 T. 7, 1888. 



